In a ruck is it legal for player on their feet to grasp a player on their own players on the ground, usually the tackled player, to prevent themselves from being driven backwards.
The laws indicate that players in a ruck should bind with a full arm, also players on the floor are deemed out of the game.
Horace LetchfordHi Horace
If a player from the ball carriers team latches onto him as described we don't have a ruck, we just have a tackle situation.
A player with his hands on a player on the floor is not off his feet, unless his weight is all on his arms rather than his feet. The rule of thumb is that if his arms are knocked away, would he fall over? If he would, then he is deemed off his feet. Also if he is kneeling on the player on the floor, he is also off his feet, because again, his weight is not all on his feet.
If a ruck has formed then you are right, that a player needs to bind fully to be part of the ruck, but what you describe usually happens before the ruck has formed. An opposition player then binds to him and drives him off the ball. Sometimes as described he drags the ball carrier with him. If the ball carrier holds onto the ball during this scenario, then he is liable to penalty for holding on.
A ruck is a phase of play where one or more players from each team, who are ontheir feet, in physical contact, close around the ball on the ground. Open play hasended.The Rugby Ref