Mahin has an excellent article on the physical processes which are engaged when we first learn to shimmy, which of course applies to learning any new moves.
“The Engine That Stalls
Remember back to your early days, instructors! New students, yes, we’ve all been where you are. You get your shimmy going and a few seconds later your knees mysteriously stop by no will of your conscious brain, You pause, then you start it back up again like an old car that stalled at a stop light. What is happening and how do you “fix” it?”
See the full article here: http://mahinbellydance.wordpress.com/2012/04/24/the-science-behind-learning-shimmies/