VERY interesting topic, zaza, thank you for bringing it up.
Some of the sexual interests that my partner and I share, involve being hyper-aware of the body language of the person you are with. When you put energy into another person (give them stimulation), it ALWAYS comes back out somewhere, you just sometimes have to watch a little more closely. For partners who are a little less vocal than others, it almost always will be evident in the feet (or legs, sometimes the hands, but usually the feet.) That's one of the reasons I'm fairly insistent that sex partners not have any footwear at all during our activities (although I do make some allowances for combat boots...)
The toe-curling and leg jiggling are sure signs that something is going on. I love it!