The rounded arc a horse makes over a fence, lifting back and neck instead of going across flat and stiff. It only happens with a free poll.
The horse makes the bascule, not you — your job is to stay out of its way. That's where the most common error sits: ride the fence in a full seat and you sit right in the middle of the arc and flatten it. Hence the light seat, weight shifted from the seat bones to the thighs, knee and heel down, stirrups shorter than on the flat. The same principle governs the hand: the long crest release, with the hand travelling a third to halfway up the neck, gives the neck the room it needs to round, and forgives balance errors best. What often gets missed: during the flight phase the horse follows a fixed trajectory. Rein aids can only disturb it there — the jump is decided before take-off.