"If you buy that tool, for-go the recommended air hold to keep the valves in place. What i do is remove all the spark plugs. So the engine is easy to turn over by hand. Stick a piece of rope into the plug hole and slowley turn the engine over (by hand, not with the starter) untill the rope is compressed in the cylinder. (make sure your on the cycle where both valves are on the seats) then you can remove the retainers/ keepers/ springs with out the valves falling down, andas a bonus you have a soft cushin when you need to tap the top of the tool to break the keepers loose."
I do the rope trick, one time though it got knotted in the cylinder, boy did I have to pull hard.