As long as the players are retiring and making no attempt to tackle, they aren't offside at a quick tap. They can't become involved in the play until they have retired 10 metres from the mark though.
For example if Team A is awarded a penalty and it's taken quickly and there is a player from Team B a metre in front of the mark, that player from Team B isn't offside if the player from Team A with the ball runs into him - as long as he is retiring from the mark.
- When a penalty or free-kick is awarded, the opposing team must immediately retreat 10 metres towards their own goal line or until they have reached their goal line if that is closer.
- Even if the penalty or free-kick is taken quickly and the kicker’s team is playing the ball, opposing players must keep retreating the necessary distance. They may not take part in the game until they have done so.
- If it is taken so quickly that opponents have no opportunity to retreat, they will not be sanctioned for this. However, they may not take part in the game until they have retreated 10 metres from the mark or until a team-mate who was 10 metres from the mark has moved in front of them.
http://laws.worldrugby.org/?law=20