I've read the rules at irblaws but I'm still confused: over there the most binding condition stated is "A player must not take a quick throw-in after the lineout has formed", but quite often it appears like it can't be done if just a single opposing player is "nearby". What is eluding me?
Your question has been answered above quite correctly. The following points are guidelines as to when a quick throw can be taken.
A quick throw is allowed to be taken if all of the following apply:
- The ball that went into touch is the same ball used for the quick throw
- The ball has not touched any other person, other than the player about to throw the ball back into play
- The ball is being thrown in by the same player who retrieved the ball (i.e a team mate hasn't gathered the ball and passed it to the player who is going to throw the ball back into play)
- The player throwing the ball back into play hasn't stepped onto the field of play before the throw is complete (i.e hasn't put part of their foot or feet into the field of play as they are in the act of throwing the ball in)
The lineout is deemed to be formed when two or more players from each team have assembled within 5-15 metres of the throw-in location.
Any player who approaches the touchline is presumed to be part of a lineout.
Up to this point, the thrower can opt to take a quick throw-in. Tactically however, and as gbianchi comments, the thrower is unlikely to opt for a quick throw in if there is an opponent in the vicinity.
Legally, however, there could be only one opponent and one team mate near the throw-in position and a quick throw would be lawful under the rules of the game.