Release. It's not enough to be on your feet.

Keegan Daniel of the Sharks charges. Juan de Jongh of the Stormers tackles him and brings him to ground. Jaque Fourie is there, on his feet, not a tackler. He has hands on Daniel and keeps them there as he leans on him.

Fourie is clearly wrong in terms of law.

Law 15.6 (c) Players in opposition to the ball carrier who remain on their feet who bring the ball carrier to ground so that the player is tackled must release the ball and the ball carrier. Those players may then play the ball providing they are on their feet and do so from behind the ball and from directly behind the tackled player or a tackler closest to those players’ goal line.
Sanction: Penalty kick