Yes, back kick is not a "spinning" kick-it must be delivered in a straight motion otherwise energy is wasted and poorly directed. I teach both a "mule" kick-like brickshooter describes-which is totally defensive.....and a more traditional kick where you look over your shoulder at the target and land your kicking foot in front keeping your momentum going towards the opponent. The key to both is keeping your body centered over your base leg and turning on a axis like you are kabob not a spinning top. Same principal applies to wheel kick (reverse round house, spinning hook kick, whatever you call it in your style).