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In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

The Guidelines for International refereesGuidelines for International referees don't mention all these points, but might give some clarification:

http://www.fivb.org/EN/Refereeing-Rules/Documents/FIVB_VB_Refereeing_Guidelines_and_Instructions_updated_03.03.2016.pdf

12.7: The 1st referee should pay attention to screening during the execution of the service when a player or group of players of the serving team, waving arms, jumping or moving sideways or by standing grouped, prevent their opponent from seeing the server and flight path of the ball (i.e. both criteria need to be satisfied for player actions/positions to be judged as a screen).So if the served ball can be seen clearly throughout its path, until it crosses the net to the opponent, it cannot be considered as a screen.

In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

The Guidelines for International referees don't mention all these points, but might give some clarification:

http://www.fivb.org/EN/Refereeing-Rules/Documents/FIVB_VB_Refereeing_Guidelines_and_Instructions_updated_03.03.2016.pdf

12.7: The 1st referee should pay attention to screening during the execution of the service when a player or group of players of the serving team, waving arms, jumping or moving sideways or by standing grouped, prevent their opponent from seeing the server and flight path of the ball (i.e. both criteria need to be satisfied for player actions/positions to be judged as a screen).So if the served ball can be seen clearly throughout its path, until it crosses the net to the opponent, it cannot be considered as a screen.

In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

The Guidelines for International referees don't mention all these points, but might give some clarification:

12.7: The 1st referee should pay attention to screening during the execution of the service when a player or group of players of the serving team, waving arms, jumping or moving sideways or by standing grouped, prevent their opponent from seeing the server and flight path of the ball (i.e. both criteria need to be satisfied for player actions/positions to be judged as a screen).So if the served ball can be seen clearly throughout its path, until it crosses the net to the opponent, it cannot be considered as a screen.

2 added reference to refereeing guidelines
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In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

The Guidelines for International referees don't mention all these points, but might give some clarification:

http://www.fivb.org/EN/Refereeing-Rules/Documents/FIVB_VB_Refereeing_Guidelines_and_Instructions_updated_03.03.2016.pdf

12.7: The 1st referee should pay attention to screening during the execution of the service when a player or group of players of the serving team, waving arms, jumping or moving sideways or by standing grouped, prevent their opponent from seeing the server and flight path of the ball (i.e. both criteria need to be satisfied for player actions/positions to be judged as a screen).So if the served ball can be seen clearly throughout its path, until it crosses the net to the opponent, it cannot be considered as a screen.

In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.

The Guidelines for International referees don't mention all these points, but might give some clarification:

http://www.fivb.org/EN/Refereeing-Rules/Documents/FIVB_VB_Refereeing_Guidelines_and_Instructions_updated_03.03.2016.pdf

12.7: The 1st referee should pay attention to screening during the execution of the service when a player or group of players of the serving team, waving arms, jumping or moving sideways or by standing grouped, prevent their opponent from seeing the server and flight path of the ball (i.e. both criteria need to be satisfied for player actions/positions to be judged as a screen).So if the served ball can be seen clearly throughout its path, until it crosses the net to the opponent, it cannot be considered as a screen.

1
source | link

In practice, the trajectory of the ball is taken in consideration as well, i.e. both the ball and the serving player must be blocked before action is taken. And considering that the ball trajectory starts quite high wit a jump serve, most referees will not interpret this as the ball being blocked from sight.

So only when...

  • the service is not a jump service
  • the ball goes straight over the screening (front) players
  • the front players seem to deliberately construct a screen (e.g. are moving along with the receivers)

...will most referees consider a screening, but as this is all very hard to see/judge from his position, most will also wait for complaints from the receiving team, and most likely solve it with a verbal warning to the screening team instead of a penalty.