Used to be one on my Kyosho TF2. It was pretty fantastic to use, I still have all of the parts. But there was a 1 way bearing involved which was it's major drawback. It was Extremely
delicate, if your car rolled backwards at all
(ie a spin, even if it was only for 5-10cm rollback), all the rollers inside the bearing broke, the bearing was set inside a spur gear hub.
My local hobby shop could never get the hub by itself, and it cost $170AU for a new 2speed. I did it twice and then never bothered again