Keep your fingers and thumbs out of the steering wheel when you do this. The front axle engages when the rear tire achieve a 6% spin. Then, presto chango, the front tires are spinning. If the wheel is turned, one tires gets good traction or something like that. Then the steering wheel will spin in your hands. Thumbs up and pinkies down will save you.
The theory is that the front will engage before the rears start hopping around. If your rears are hoppin around and spinning with the front never even engaging. Then you need to download the pre 1965 -20 manual for the M35. It walks you through how to adjust the sprag and how to test it. The official test does not include a tree or spinning the rear tires by the way.