You should have the air filter off and a large wad of rag at the ready when you're doing this, that engine may eventually get enough compression in a couple cylinders to fire off and when it does if the rings are stuck it can very easily and quickly start dieseling on the crankcase oil and run away on you!

I had a similar situation and the only way to stop it is to cut off the air supply, the engine I was fooling with could be throttled using a large wad of rag restricting the air intake.