This is a neat way to insert a rotary group into an axial piston pump or motor with the pistons pointing down. Very often there's not enough space to hold the pistons and the retainer plate with your fingertips, and since the pistons tend to fall out of the barrel, the insertion process may become somewhat "awkward", forcing you to put the unit on its side and insert the rotary group assembly in the horizontal position.
A simple o'ring (adequately large, of course) arranged in a tying loop can be of great help here - creating an elastic retainer that puts inward pressure on the pistons and does not let them slip out of the cylinder block. The pictures are pretty self explanatory. Just don't forget to take the o'ring out after the rotary group is in place!