If I am a defending player and a ruck is formed, at what point am I/we allowed to touch the ball with our hands?
Hi Phil, thanks for the question.
As you have rightly pointed out, once a ruck has formed you cannot use your hands to win the ball.
16.4 (b)Players must not handle the ball in a ruck except after a tackle if they are on their feet and have their hands on the ball before the ruck is formed. Sanction: Penalty kick
So when can you start using our hands again?
Well, not until the ruck is over, so when is that?
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.
The usual question here that referees are asked is "when has the ball left the ruck?" The law doesn't actually say, which means (like many laws in Rugby Union) it is up to the referees judgement.
The Rugby Ref says the ruck is over when the ball leaves the ruck, which means the confines of everyone involved in the ruck. Imagine stretching a rubber band around all the players in the ruck; that is the players on their feet competing for the ball with their feet, and the players on the ground unable to move away, or play the ball. If the ball is inside this imaginary rubber band, then the ball is still in the ruck and can't be handled. Once the ball pops outside the rubber band, the ruck is over and can be picked up by any player in a legal position to do so.
So Phil, the short answer to your question in "not until the ball leaves the ruck".
The Rugby Ref