Bear in mind that we want the whole valve mechanism to be under the control of the camshaft at all times. The cam doesn’t push the valve open and then the valve spring snaps the valve shut; the valve would never be able to tolerate that kind of violent seating. Here’s what has to happen: The mechanism accelerates the valve off its seat, the valve reaches the top of its lift and stops. The spring, holding the valve against the cam profile, reaccelerates the valve back toward its seat. As the valve nears the seat, the cam slows the motion and the valve goes back onto its seat at a survivable speed of, say, 2 feet per second.