Usually a horse gives some warning that it is about to buck by dropping its head, slowing down or stopping, and excessively rounding up its back. With such an advance warning, riders can intervene in early stages by encouraging forward motion or circling.
A horse typically gives notice of a possible buck by dropping its head, slowing down, or halting completely, and rounding its back sharply.  Riders can then act quickly to prevent a full buck by urging the horse forward or circling it.