[LAWS]DEFINITIONS
A ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground. Open play has
ended.
Players are rucking when they are in a ruck and using their feet to try to win or keep possession of the ball, without being guilty of foul play.[/LAWS]
Unfortunately, these definitions seem to no longer accurately describe a ruck
The whole intent of the ruck was that it was an "informal" (as in no specific rows of players with individual binding Laws) scrummage that started with the ball already in it. The basic operations relating to the ball in the ruck are the same as those in the scrum
the ball is on the ground
players move the ball with their feet
players aren't allowed hands on the ball
The whole reason for the "hindmost foot" concept was that the ball would be competed for by using feet, and that the last (hindmost) player in the ruck would move the ball back to the acting scrum-half by rolling it back with a foot, and when he did that, the ball would pass behind his hindmost foot. The ball was considered out at that point.
Unfortunately, as the iRB has made the use of feet in the ruck more and more difficult, so the ruck has progressively become more and more of a disorganised shamozzle, with players off their feet and players lying all over the ball. However, there are a couple of things that we can say for certain...
1. The tackled player and any tacklers, if they are on the ground, are not supposed to be part of the ruck. How can they be, when they aren't even supposed to be there at all...
[LAWS]15.4 THE TACKLER
(b) The tackler must immediately get up or move away from the tackled player and from the ball at once.
15.5 THE TACKLED PLAYER
(b) A tackled player must immediately pass the ball or release it. That player must also get up or move away from it at once.[/LAWS]
2. Players who subsequently go off their feet are no longer part of the ruck.
[LAWS]16.2 JOINING A RUCK
(d) All players forming, joining or taking part in a ruck must be on their feet.
[/LAWS]
3. Even if all the players taking part in a ruck go off their feet after a ruck has been formed, the ruck still exists..
[LAWS]16.6 SUCCESSFUL END TO A RUCK
A ruck ends successfully when the ball leaves the ruck, or when the ball is on or over the goal line.
16.7 UNSUCCESSFUL END TO A RUCK
(a) A ruck ends unsuccessfully when the ball becomes unplayable and a scrum is ordered. The team that was moving forward immediately before the ball became unplayable in the ruck throws in the ball. If neither team was moving forward, or if the referee cannot decide which team was moving forward before the ball became unplayable in the ruck, the team that was moving forward
before the ruck began throws in the ball. If neither team was moving forward, then the attacking team throws in the ball.
(b) Before the referee blows the whistle for a scrum, the referee allows a reasonable amount of time for the ball to emerge, especially if either team is moving forward. If the ruck stops moving, or if the referee decides that the ball will probably not emerge within a reasonable time, the referee must order a scrum.[/LAWS]
Nothing there about the ruck ending when all the players go off their feet, and this brings us to a curious anomaly in the Laws. It is entirely possible to have a ruck... with no players in it!!!
Now, I am sure that the Lawmakers never intended this. Its a great example of the "Law of Unindented Consequences"; the effect of removing the use of the feet to win the ball in a ruck has led to the possibility of having a ruck with no players in it.
So how do we deal with this.
Well, for one thing, since we have a ruck in the first place, at least one player in each team must have been on their feet at some point, so we can say
► The hindmost feet of those two players will set the offside lines, and be the determining points of when the ball is in or out.
► If any players join the ruck, the offsides and determining points may be reset to their hindmost feet.
The problem comes if any of the hindmost players go off their feet, do their feet still define the offside lines and positions where the ball is deemed out?
If the tackler and/or tackled player are not on their feet, then they are not part of the ruck; they never were, so the position of their feet is irrelevant. However, the position of the feet of any players who were in the ruck and who subsequently went off their feet, are relevant, and that brings us to yet another problem. In the shambles of bodies lying all over the place, it is not always possible to tell which legs belong to which players, so quickly determining if the pair of feet sticking out the back of the ruck belong to he tackler, the tackled player, or a ruck player gone off his feet, is not always possible.
IMO, this is one of those rare occasions in which Occam's Razor doesn't apply; the simplest solution is to treat ALL feet as the same, but it probably isn't the correct answer.