When is it not a ruck?

I have played and/or coached rugby union in Scotland and the US since 1974. I am currently an assistant coach for a U19 men's team. 

The definition of a ruck includes "...close around the ball on the ground."  So how close is close? How far away is no longer "close?"

I often see a first offensive rucker fly in "through the gate" going 1 or 2 or more meters past the tackled player (and ball) to "clear the threat" and then binding (actually tackling) a defensive player who may or may not be advancing "through the gate" on his side. Is that a ruck? When is it illegal, perhaps a violation like tackling a defensive player who does not have the ball or maybe obstruction?

Particularly in this setting where the first offensive rucker is 1, 2 or more meters away from the tackled player & ball, if the first defensive rucker simply escorts/allows or even binds and pulls that first offensive rucker backwards a few more meters down the pitch away from the tackled player & ball, when can a second defensive rucker just step into the wide open space to "enter the gate" on his side to either contest for the ball and/or try to unseat the "sealer" (second offensive rucker)?      

Claude Hughes

Chapel Hill Highlanders

Hi Claude

"Close around the ball on the ground" means the ball has to be in between them in some way.  It's close around, rather than close to (as in near to). Generally we would expect the ruckers to be over the ball as shown in the diagram in the law book.

If an attacking player went beyond the ball before the ruck formed then he would potentially be in an offside position (ahead of the player from his team who last played the ball) and we would expect him to move away and not obstruct.  This is often referred to as "taking space".
If the ruck is formed we have offside lines and if the ruck is then driven off or beyond the ball, then the ruck is over.  We are back to open play and anyone can step in and play the ball, or potentially form a new ruck.

Thanks for the questions
The Rugby Ref