Chicken is a game mechanic in Tekken that allows a player to turn opponent's Parry#Reversal against them, and deal damage to them as a result of a succesfull chicken.
Chicken is done when opponent starts to reverse your move by pressing f+1+3 or f+2+4 depending on what limb you used to attack. You press f+1+3 to perform a chicken if you attacked with your left limb and you press f+2+4 if you attacked with your right limb. For attacks made with limbs on both sides of the body(left and right hand, for example) it varies and you will have to chicken in with either f+1+3 or f+2+4 depending on a move.