Tackle infringement.

Brock James of Clermont Auvergne gives to Julian Bonnaire and the big man forges ahead but Pedrie Wannenburg of Ulster hauls him down. Having tackled Bonnaire, Wannenburg then scoops the ball back to his side.

The referee penalises Wannenburg.

He has two good reasons to penalise Wannenburg.

Firstly Wannenburg did not allow time between bringing Bonnaire to ground and playing the ball. There was no time for Bonnaire to discharge his duty of releasing the ball.

Secondly, Wannenburg was not on his feet  while playing the ball in that he was not supporting his own weight.

Law 15.4 THE TACKLER
(a)  When a player tackles an opponent and they both go to ground, the tackler must immediately release the tackled player.
Sanction: Penalty kick
(b)  The tackler must immediately get up or move away from the tackled player and from the ball at once.
Sanction: Penalty kick
(c)  The tackler must get up before playing the ball and then may play the ball from any direction. Sanction: Penalty kick

Law 15.6 (a) Players are on their feet if no other part of their body is supported by the ground or players on the ground.
Sanction: Penalty kick

James (22) does come in the side but by then Wannenburg had already infringed.