So if Blue 5 ran into the Blue 6 position and stopped, would he be able to recontinue into the ruck or would he need to retreat back to the Blue offside line first? I guess that technically he is offside once he stops at the Blue 6 position and is subject to penalty.
It hasn't occurred to me before that a player's offsidedness may be influenced by his motion/lack of motion.
Its more a dynamic situation than a static one
AIUI the intent of this law isn't so much to ping the player who is standing offside who then joins the ruck, it more to force the retiring player (who is perhaps coming back from a previous play) to go all the way back to the offside line before joining.
Take the example of a midfield break by Red.
When the Red player is tackled, gates appear immediately. Blue players are retiring, and must go through the gate if they wish to play the ball.
Then a ruck forms. The gates disappear and are replaced by the hindmost foot offside line. If there was no requirement for slow blue players who are still retiring to go back to their HMF offside line before joining, then they would gain a significant advantage by their slowness.
IMO, it is not so important to be strict about side entry for players who have already retired to the HMF offside and decide later to join. Their offence is less material than the one committed by the still retiring player. In fact, if a player is slightly offside, I would rather see him join the ruck than remain where he is. If he remains, he is liable to penalty.
[LAWS]11.9 LOITERING
A player who remains in an offside position is loitering. A loiterer who prevents the opposing team from playing the ball as they wish is taking part in the game, and is penalised. The referee makes sure that the loiterer does not benefit from being put onside by the opposing team’s action.
Sanction: Penalty kick at the offending player’s offside line[/LAWS]
IMO, players are not PKd under this law often enough. Loitering players may not be having an immediate material impact on the game, but they are, by their very presence, limiting the opposing team's options to "play the ball as they wish" and might very well have an impact on what is going to happen next.