Understanding the dynamics of the technique you use.
What causes speed? Correctly timed acclerations and deaccelerations of the twisting the hips, bending legs, and bending of the feet.
Do this: Sit in a chair that can spin. Try to spin around as fast as you can. When you are spinning in the chair, experiment with the positioning of your body and your limbs. Know what slows you down, and what speeds you up. This will help you better understand how to go faster when performing the windmill.
Yes flexibability helps, but it only helps so much. The rest is up to you...
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Have a good OnE.

KAIZEN- Constant self-improvment.
Perfect work -> Perfect Results
Oustanding work -> Oustanding Results
Excellent work -> Excellent Results
Great work -> Great Results
Good work -> Good Results
Poor Work -> Poor Results
Nothing
^ None of that chart is true. The truth is that Perfection in work gives you Outstanding results. It works that way for all levels. If your do a poor job at work you will get fired, and have nothing.