Clip 1 - 13 September 2017 - Law 20
Collapsing under pressure.
Stefan Ungerer of the Pumas puts the ball into a scrum. The Pumas get possession of the ball but the Sharks get a shove on and the scrum wheels and falls down.
The referee penalising, saying: "collapsing under pressure."
There is no such penalty in the law book just as there is no penalty for "standing up under pressure".
There is a penalty for collapsing a scrum.
Law 20.9 Scrum - general restrictions
(a) All players: Collapsing. A player must not intentionally collapse a scrum. A player must not intentionally fall or kneel in a scrum. This is dangerous play.
Sanction: Penalty kick
There is no "under pressure" part, though referees use it.
There is pressure in a scrum. Those who feel it most are usually those in the front row. Sometimes there is pressure on the neck of a player in the front row, which can be seriously dangerous. To protect himself the player may stand up in the front row - which is not an infringement - or go to ground, which is an infringement.
It would be best to leave "under pressure" out of any reason to penalise.