There are two versions of this.

1) Two people are fighting, and although it's perfectly logical to stab a person in his chest or hit him in the face, instead the he gets a knife/sword/blunt object slammed into his foot. This, of course, is far more effective, because we all know that the feet are a person's largest weakness.

2) Antagonist is holding a hostage, and the hostage stomps on his foot, which makes him let go, because we all know there's no pain in the world like a foot smash.

Also.. I'm a poor writer, but I think you get the jist.