Twist kick is similar to reverse crescent except it is hitting with the instep (top of the foot) instead of the outside blade of the foot. Also, it chambers similar to a front kick.
I also strike with the heel for side kicks. I think it is much more effective for both sparring and breaking (power). However, I do see some in TKD use the blade/knife edge of the foot, often when doing forms. I think it depends on your target as well -- if your target is the throat then I expect the knife edge would be more effective than the heel.
-Chris
I just looked at the description again, and I think we actually do this kick and call it an inside roundhouse. I really dislike this kick; not for a good reason, though. I am very pigeon-toed due to a minor birth defect, and the inside roundhouse is just not comfortable for me to do. My understanding is that it's a really good kick if you are in open stance with your opponent and want to strke quickly to the body or groin. Not that I can do it with any kind of speed or accuracy myself. Thanks again for the information.