It is important to get the engine to TDC compression stroke for the cylinder you are working with otherwize you will blow the piston down to a point where the valves open. Bring the piston to TDC compression stroke using your distributor or you thumb on the plug hole as a guide. If you can see the piston get it all the way op top by eye. You will have to feel for the exact TDC by rocking the crank back and forth to feel for the piston going over center. It's a very suttle feel. At this point the piston will have no leverage to rotate the crankshaft when you apply pressure. Also 100 PSI is high, you might be better off reducing pressure below 40 PSI just in case you are slightly off TDC.

John Roncallo