Look down into a glass of water as it turns. The glass moves at once, but the water and its bubbles lag behind. That lag is how the semicircular canals sense head rotation.
Nothing moves relative to the glass, so the flap on the wall stands straight.
The water follows the glass through friction at the wall: its speed closes the gap to the glass's speed with the time constant set above (water near the wall catches up a little faster than water in the middle). The semicircular canal works the same way, with the endolymph as the water and the cupula as the flap; its time constant is about 6 seconds. The canal is a thin ring of fluid rather than a full glass, and its cupula also springs back, but the lag-then-catch-up-then-overshoot story is the same.
Part of the VOR Simulator by Michael Sanders · See also the Labyrinth Model Lab