If you're driving you car and turn one direction you will feel centrifugal force move you in the opposite direction.
Same thing on a bike; you turn left, centrifugal force exerts right. The bike, being balanced on two wheels leans right and moves that direction. It doesn't take much of a counter-steer to effect that movement and once you're leaned, more counter-steer leans you further.
It doesn't work at low speeds due to the lack of gyroscopic effect to which Greybeard referred.
