Scuba unit removal and replacement
The picture shows a nice Belgian variation of the original PADI exercise: A group of divers is using only two scuba units underwater. This is very exciting because some divers have to wait before they get the regulator for breathing.
During the PADI exercise you only remove your own scuba unit for one moment and then you replace it. You shall train this to get used to the handling of your gear underwater and to be able to quickly remove and replace it, if a real situation would require this action.