If the ball is on the other side of the ruck and appears to be out, you arte allowed to contest it, as it is out of the ruck. Howver i would recommend talking to the ref before the game to establish when he considers the ball to be out of the ruck. Some refs I have had believe the ball is out of the ruck from the moment the ball is touching the opperstions hand, other believe that it is out of the ruck when the ball sees day light. You must make sure you follow when the ref considers the ball is out. If the ball is down on the ground, and a ruck has not formed, where there is a person from the other team contesting the ball then it is a ruck, however if the tackler is down on the ground, and noone from his team is in support, then that ball is yours, so you want to stay on your feet and grab the ball. If the tackled players attempts to hold onto the ball, he will be panalised for not releasing.