[Gordon Rankin]

Normally the closest the piston and the valve get is when there is combustion in the cylinder when they are in "sync". At this point the valves are "closed" or fully distended and the piston is extended to the max. But when the valves and the piston are out of "sync", the valve is fully extended and the piston is likewise fully extended. Sorry, but this is as simple as I can get it..And is just another way of explaining what Looneybin explained above.

Remember that it is called a timing belt for a reason, to "time" the explosion and exhaust of gases. If the "sync" is not timed, the piston wants to compress at the same time the valve wants to open or exhaust.