Valve bounce it exactly what it says. The valve actually bounces. The spring is not strong enough to hold shut the valve in one hit at high revs/ high inertia. The valve flys back up, hits the seat and bounces. Only marginaly but enough for a terrible noise and power loss. It is also damaging to the head/seat and the valve.

Basically if you dont have valve bounce or arent on the verge of it at your motors peak rpm dont worry. Save your time and money for something else.