Ushiro-geri is the Japanese term for back kick, a kick employed in taijutsu. Back kick is the most powerful out of all the basic techniques. A more advanced variation of back kick is the spinning back kick.
How to execute a back kick
Step One: Start facing your target in sparring stance (body sideways). Look over your shoulder (the shoulder on the same side as the leg that's in the back, as this is the leg you will be kicking with).
Step Two: Turn your feet so your toes are pointing the opposite direction they were before (pointing to the back rather than the front).
Step Three: Though you have your back to the target, make sure you are looking at your target over the same shoulder as in Step One. Kick straight back, pushing out with your leg and aiming with your heel.
Step Four: Re-chamber your leg and land your kicking foot in front, moving towards your target.