Stagger Kick, called Alley Kick (アリキック/Ari Kikku) in Japan, is an attack used by the Kings from the first game and onwards. In the first game, its input is f+4,4,4 while crouching. In later games (at least from Tekken 5), it gains an optional d+3+4,4,4 or WR 4,4,4. If it hits as a counter hit, an extra 4 input can be used to extend the attack.
It is also a part of Roger Jr.'s move starting with Tekken 5. This version's input is WR 4,4,4; which is also another optional input for the Kings.