This is very similar to something I tried when I was a kid (see the OOBE story). One can deliberately get this sensation by closing one's eyes while sitting down and imagining oneself on a rollercoaster or wobbling around one's body-axis like a see-saw. If you concentrate on this idea, you may actually begin to feel your body move and tumble. Its quite unsettling. I think it has something to do with hypnotizing yourself into a trance like state, something that might happen unconciously while sitting at a stop-light or in front of a computer.