And they can come in low under the radar, SLAM into the front of the knee or groin, even if the guy is pretty close up behind you. Probably he won't even see it coming.
Make sure your hips stay squared and that kick rises up to chamber and fires straight back, striking with the bottom of the heel, toes pointed down and pulled back to keep the heel extended. Then retract the foot to the chamber position and plant it down next to the other foot, right where it started. Make the plant with a bit of authority, to make sure you stabilize yourself and you aren't tottering off balance or something. That is a good basic way to practice the technique.
If you turn your body too far to look, then you rotate your hips and it becomes closer to a side kick. I also love a side kick, but in this case, the back kick is more powerful and the position of the hips is more stable. Literally, you should only peek over your shoulder and just see him in your peripheral vision. If you look to see him fully, you are turning your body too much and it becomes a side kick.
Good advice, Michael. Part of my problem may be the tendency to turn my upper body too far in advance of the kick to seek out the target; in doing that, you wind up misaligning yourself for the straight-back motion that the kick is supposed to consist of. It's one of those damnable little wrinkles that can take a long time to work out.
I really appreciate all this shrewd input from you more-experienced back-kickers!