The exercise is the same as with a regular bench:

Take two dumbbells and lay on a bench, make sure the feet are firmly on the ground and your back is not arched, but has good contact with the bench. The arms are stretched in front of you, about shoulder wide. Bend now the arms a bit and let them down with a half-circle movement to the side. Without changing the angle of the elbow bring them in a fluid movement back up.

El diagrama muestra los músculos más usados en este ejercicio