My view is that it is so fundamental they put it in the Law book twice.
[LAWS]15.7 Forbidden practices
(e) Danger may arise if a tackled player fails to release the ball or move away from it immediately, or if that player is prevented from so doing. If either of these happens the referee awards a penalty kick immediately[/LAWS]Didds, I am surprised a coach should ask the question. The law is there to safeguard the player's safety. Being off his feet he is out of the game. Since the game is about competing for the ball, the longer he remains near the ball while off his feet, the greater the risk of getting injured by arriving players. Or is your question about some other aspect of 15.5?
[LAWS]15.5 The tackled player
(b) A tackled player must immediately pass the ball or release it. That player must also get up or move away from it at once.
Sanction: Penalty kick
By the way
LoTG provide 3 video examples of this being enforced.[/LAWS]