- Stand on something that allows your feet to slide/ move around freely. You can stand on Valslides, put your feet into gymnastics rings or stirrups of a suspension trainer, or even wear socks on a very slick linoleum/ hardwood floor. The less friction the better.
- Let your feet slide out to the sides into a straddle - as if you're trying to do a split. If you can do a split, then great. If not, go as deep as is comfortable. If you need to hold onto something for balance, do so, but don't hold yourself up with your hands. Also, it helps to tilt your pelvis forward (anterior pelvic tilt) - as if you're sticking your butt out - to clear space in your pelvis.
- Once you've gone as deep as you can, hold briefly, then pull yourself back up using just the strength of your hip adductors (the muscles in your groin and along your inner thighs).
- Repeat. Do up to 5 reps. As you get stronger, try going deeper, or holding some weight while you do it.
This exercise is specifically for karateka or other martial artists. It's designed to build up strength in your adductors with your legs spread far apart. To be honest, this isn't a range of motion that is important for most athletes - you don't really ever see football players in that position, at least not on purpose!
But if you want to have good high kicks (and don't we all? Regardless of how impractical high kicks are for self defense?) then you need to be both flexible and strong in your hip adductors - so your kicks are high and you remain stable in the high kick position. Very few exercises target these muscles in this range of motion.
Do Straddle to Stands twice a week. Combine them with daily dynamic stretching and some evening passive stretching (while cold) and you'll be kicking people in the head in no time!