you are right the term "double movement" is not mention in the rugby union laws. But once you've placed the ball and you're held in the tackle on the floor you must release. Therefore, if you are tackled, held and reach out to place the ball across the try line and do not quite reach it, you must release the ball in that possition. You are not allowed to then place the ball back toward your team, or scoot forward and place the ball further down the pitch. So while the term "double movement" isn't mentioned in the law book, it is a shortened way to say what I just said. No Try. »
Legitimate question, not trying to be sarcastic. Can someone explain how that obstruction is different than every other movement when "dummy runners" obstruct defenders. It happens constantly. Why was this called. Am I missing some nuance in the law?