Kosoto-gari can be used to reap an opponent's leg by either
by bringing the back of the opponent's knee onto your inner thigh to
destabilize his balance, or by scooping up his heel.
The "back of the knee" method is performed from a Kenka yotsu (Asymmetrical grips by the two opponents) stance by Tori (the player executing the technique) stepping toward the opponent with what will be his Support leg.
At the same time, Tori (Player executing technique) bends his knees and
pulls his lifting hand downward to lower his center of gravity. Tori (Player
executing technique) then pulls the back of Uke (Player receiving opponent's
attack) knee onto his inner thigh so that Uke (Player receiving opponent's
attack), unable to back up, falls to the floor.
The "heel scooping" method (shown) is performed from a
Kenka yotsu (Asymmetrical grips by the two opponents) stance by Tori (Player
executing technique) moving immediately to hook Uke (Player receiving
opponent's attack) ankle.