Changing valve springs on car

Some ont like this idea, but I've never failed with it. I use a couple feet of cotton rope from the hardware store. bring the piston up about 1/2 before TDC on compression, thread the rope in to the spark plug hole, leaving about 8" hanging out the spark plug hole. Then, bring the piston up towards TDC. When you feel resitience to the turning, stop. The rope holds the valves closed, while you work on the springs. Some have said the rope got stuck..I think that may be from the valve catching it and trapping it in the seat. If the valves are closed tight, I dont think they'll be any trouble. WHen you're done, rotate the engine backwards to realease the rope, pull it out, and do the next one. I dont like air. I've had the engine spin so the piston is at the bottom of the bore, and the valve can be pushed in or fall in if the pressure isnt high enough..the rope works fine, and there's no air line over the car.