C.F.
My now-16-month-old has been doing this off and on since she was a couple of months old too.
She would lift her legs from the waist and just let them drop down on the mattress - whump!
It amazed me that she could do this while sound asleep - I still don't know how/why she does it.
She doesn't do it much anymore, or maybe we just don't hear it anymore. = )
Also, sometimes she'll do it while she is awake trying to fall asleep.