The perrier only hurled light rocks of up to 10lb, but could achieve a high rate of fire, perhaps 10 per minute. Rates of fire of up to 1000 rocks per hour have been recorded. When perriers were used for both attack and defence, there are stories of the hail of stones being so heavy that missiles were colliding in mid-air.
Perriers relied on simple muscle power for their action. This is a three rope example, powered by six soldiers pulling on it. There are stories of large perriers using up to two hundred soldiers.
Part of the perrier's rapid fire comes about because the engineers could stay in their firing positions as the engine was prepared for a further shot. 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.