It's rapid muscle twitching, possibly caused by your brain reacting to the beats themselves. I also get this rapid-fire twitching with the relaxation and LD binaurals, and have read other posters asking about this-at the same stage. This goes away when you get into the deeper frequencies in the later part of the program. I guess it has to do with the specific frequency the LD and relaxation presets run at.
It's nothing to worry about, it's just weird at first.