Baby Thrill-Seeking Behavior

El, 6 months old, does the strangest thing. You put her down for some naked time on a baby blanket, on the carpet. Lying on her back, she picks up a corner of the blanket and pulls it toward her, then drops it onto her face. She then splays her arms out and flaps them up and down, while her feet kick in unison, so wildly that her entire rear end is repeatedly launched into the air, and her feet slam out a staccato rhythm on the floor. During all this she's also twirling her hands at the wrists - something she's always done when excited. My description cannot possibly convey how hilarious this is to watch - not to mention disconcerting. A tiny baby madly bouncing herself off the floor, naked except for the blanket completely concealing her head. When I first saw her do this, about a week or so ago, I kept pulling the blanket off her face because I'd assumed she had covered it by accident, due to all her frantic gyrations. Plus it freaked me out. But then she kept grabbing the blanket back and dropping it over her face again, and after some closer observation I realized that she was doing it on purpose. All the excited movement was happening because she was getting a thrill from this. Since the blanket was extremely lightweight and I could clearly hear her unhindered breathing, I finally let her go with it the other day, to see what she would end up doing. She left it there for about twenty seconds, her whole body absolutely berserk with excitement. She squealed with laughter - I wish I could have gotten video. I got a glimpse under the blanket a couple times and saw that her little eyes were wide and staring at the blanket, which was no farther away from her than her nose. Does the thrill lie in seeing the blanket up so close? Or in the fact that she can't see anything else? Is this a mastery thing - scaring herself just a little bit, and getting a kick out of tolerating slight fear? Kind of like the way a very mild pain can be experienced as a tickle. I don't know. I've never seen a baby do anything like this before. What an interesting little person she is.