Easy way to determine how a joint breaks is to see the joint's range of motion and work from there. Essentially, you either wedge a part into the ankle so it overextends without ghosting into it (since the ankle is a durable joint, ghosting will usually cause a deform), hit the foot part so the ankle contracts too far (this includes smacking the toe to push the connection between the ankle and the foot too far), or hit the heel part of the ankle, where the range of motion on the joint is at it's most limited,
Since the weight distribution between the two parts that are held together by the ankle are so unbalanced (foot vs rest of body), it's typically easier to break the ankle after the leg has been detached, since it's less likely to redistribute the force to the rest of the body, and you're more able to forcefully restrain the ankle's range of motion by having a grab on the shin or knee.