Can a player rotate from the back line after the serve and become a blocker? Or is the designation of backline player at the time of the serve?
A back-row player (one of the three players in the back row at the time of the serve, including the server) is not allowed to block. If they do, it is a blocking fault and results in a point for the other team.
From the FIVB Official Volleyball Rules 2013-2016, Rule 14.6 (Blocking Faults):
14.6 Blocking Faults
14.6.2 A back-row player or a Libero completes a block or participates in a completed block.
No, he can't. It's a fault and the point is given to the other team.
Also, a back-row player can complete an attack in two ways:
- at any height from behind the front zone if at his/her take-off, the player's feet didn't touch or cros the attack line
- from the front zone if at the moment of the contact a part of the ball is lower than the top of the net.