One of the common mistakes when practicing the shotokan karate back kick or ushiro geri, is the over spin! Karateka (someone who practices karate) spin to do the back kick, but continue spinning and in some cases end up executing a cross between back kick and spinning hook kick.
When you start the back kick, you do indeed start to spin the body and hips, but the circular spin is only for the start of the kick, just the hips face the target squarely, the hips drive forward and into the target.
This exercise helps to eliminate the hip and body over spin. Simly turn and kick ushiro geri on your partner, then stop with the kicking leg fully extended. Hold for a few seconds then come back the way you came and settle back into a fighting stance.
Ushiro Geri Exercise Video