Ah..the back kick! One of my favorites!!! I use drills that draw the person into the kick such as retreating foot work or circling out as well as counter drills where I set up an attack, say turning kick or wheel kick, and counter with back kick (tHere are a lot of variations!). Beginners have a hard time throwing it due to balance problems so a favorite is to have them throw side kick then back kick to their partners hogu while the partner moves to either side after the first kick. This forces them to focus on technique and balance in order to hit their target (as opposed to just throwing it wildly behind them hoping to hit something).