 As the centre of pressure on the sidepod aerofoils moved about depending on the car's speed, attitude, and ground clearance, these forces interacted with the car's suspension systems, and the cars began to resonate, particularly at slow speeds, rocking back and forth - sometimes quite violently.Some drivers were known to complain of sea-sickness.This rocking motion, like a porpoise diving into and out of the sea as it swims at speed, gives the phenomenon its name.