[Ali Ahmed]

I can see no rule which would make this illegal but looking at how much the wheels will slide I can't see this being a useful mechanisms. I figure that it will just make it easier for another team to push you. For a real world example think about Anti-lock Braking systems; they allow the driver to maintain control over his car by keeping the wheels moving.

But, coming back to your original question, no, doesn't seem to be illegal.