A Rear Backfist Strike in the form Toi-Gye
A Rear Backfist Strike is a strike made with your backfist to an opponent standing behind you. Imagine the opponent is grabbing you by the waist from behind, and you bring your backfist up to strike him in the face of your shoulder.
This strike appears, for example, in the forms Toi-Gye and Chul-Gi.