Crankshaft won't Turn by Hand Anymore

[Sylvanite]

I was trying to get the distributor back to the right timing (see my other thread), when the crankshaft stopped being able to be turned by hand. It will become very difficult to turn after turning an inch or two, and always spring back to its previous position when I release the wrench. I thought this was perhaps due to vacuum and pressure in the cylinders, so I held the wrench in place for about 30 seconds to let the pressure equalize, but this did nothing.

It never did this before, and will go back to being able to be turned (for about an inch) if I crank the engine, but the problem always comes back. I really need this to work correctly so I can fix the timing.