I have just reached green-yellow belt in tae kwon do, and my instructors say that I need to work on my spinning side kicks. Unfortunately I have trouble getting the form right, so I usually end up off-balance or off-center. The jumping spinning kicks are even worse. How can I improve my form, and how do I know when I'm doing it correctly?
Different people will do this kick differently - when not simple incompetence it can be because - as with most martial arts techniques - there are alternatives giving a different combination of speed, lack of telegraphing, power, control, delayed commitment, recovery time, ability to defend yourself during and after the kick, reach, ease of blocking etc.. There's often no clear right and wrong way. I say this partly so that if you try what I suggest below and your instructor disagrees - understand we both may be "right" for different scenarios of use, where there are different priorities.
There can also be differences in terminology and classification. I don't consider there to be a useful "side spinning kick" distinct from a "spinning back kick" - I only use one kick in that "space".
I recommend trying to get a linear front-to-back feel to the kick, with the spin done early and minimally. For practice, if you start with say your left leg forwards in a back or "L" stance, pivot on the balls of both feet such that your front heel turns towards the target, and your back foot turns so the heel is well past forwards, giving some side-to-side stability. Keep the body weight back and the back leg bent. Do not arch your back - rather, keep a straight back and incline it away from the target, then look past your chest at the target. As you push off the back foot, pick it up with the kicking knee out to the side - not tucked under your hips close to the other knee - and keep the shin more or less on line to the target through the entire kick. The kicking hip should be held away from the target early on in the kick, then rotated powerfully towards the target. Roll over the ball of the non-kicking foot so that leg is bent but straightening towards the target during contact.
While only mentioned as an exercise for developing good hip movement in a non-spinning side kick, one of my videos on youtube does illustrate the above, starting at 1:50 but especially at 3:08 onwards in http://www.youtube.com/watch?v=uTtyTOLqnoc (no point asking me questions on there - I don't remember my login).
I'll take a stab at this for you since there isn't an answer yet....
The first thing I'll say is that in the case of a side kick, moreso than any other type of spinning kick, the spin isn't about generating extra power in the kick, it's simply about opportunity and changing the direction of chambering the kick.
For all of the following discussion assume you are in a left cat stance(Hu Gul Jaseh) with your left foot forward and pointed at your opponent. Your right foot, bearing most of your weight is back and pointed 90deg right.
So you'll be performing the side kick with your right foot.
The first thing to realize is that the chambered position will be exactly the same as in a side kick. So if you feel comfortable with a side kick, perform the motions stopping just before you begin the leg extension and realize this is the same chambered position you should be in at the end of the spin: left foot pointed back close to 180 degree from it's starting position, left leg slightly bent at knee, right leg bent and pulled up so the knee is at least as high as your belt and knee pointed to your left.
Now, how to get there.....
I'm going to break this down into steps which some may argue with. In the future you will combine the first few steps into one motion, but the transition is instinctive once you get the feel of the mechanics.
I'll just modify my answer from the linked question:
I am a TKD instructor. That, and I used to do Latin. Ballroom, HipHop and Freestyle dancing. The combination of these things helped me pick up the techniques fairly intuitively, but the point is that you need to practice kicks in stages: first learn to spot when you do a turn, in other words: don't swing your head, Keep your head on the target like you have a built-in gyroscope. I.e. lead with your head. When you've got the balance/spot thing sorted, practice a regular side kick. When you've got THAT down, practice spinning your knee and stopping in a controlled manner. Finally, practice the actual spinning back kick, taking care to end in a controlled manner. When THAT's taken care off, try hitting a small-ish target like a tennis ball on a piece of string.
The thing that puts you off-balance is your head being out of alignment. You need to keep your eyes focused on the target. Otherwise you lose your balance and either the kick doesn't count, or you get disoriented and get a mouth full of your opponent's foot.
Practicing in a swimming pool will probably help a bit, but it's more important to learn the various parts and piece them together properly. After that, getting to perform the technique in extreme slow-motion is not a bad idea.