A loader would sit in a perilous-looking position within the frame of the perrier, loading a new stone as soon as the arm swung back down. The perrier's ammunition is carried in a rope basket, fastened to a sling on the throwing arm. One end of the sling is fastened and the other is passed over a hook. As the arm passes over the topmost position, the sling unhooks and the rocks fly free. Like most of the over-arm engines, the perrier has a high trajectory.
Stones arrive crashing vertically downwards.
They would often be used, not against a stone-built castle but against the timber and thatch buildings within the castle's outer walls. Incendiaries might also be used.