I'd been pondering this one for the last few weeks, and it is such a strange sensation....
I'd head it was like a butterfly or indigestion or bubbles, but that just didn't seem to describe it to me. I think I've cracked it, though! If you've ever held a weeny frog in your hands, you'll know what I mean - it's like when that frog jumps. Kind of soft and jumpy, not like a cricket or a grasshopper, but a soft baby frog!
Or perhaps a little fish, flipping over.
Am I going insane or does anyone else agree?!!