We practiced our kicking this session. We started with mae geri, raising the knee (keep the foot tucked in) and snapping the foot forwards and back quickly, kicking with the ball of the foot.
Yoko geri keage - side snap kick, is similar in that it snaps out and back quickly. The direction is to the side and you kick with sokuto - the outer edge of the foot. To ensure you kick with the right part of the foot, you need to turn your hips (like a pendulum). This kick has an upward direction too it as well.
With snap kicks, the knee remains in place. With thrust kicks, the knee moves in the direction of the kick, thrusting out.