If holding the plane stationary and pointing the nose down while at throttle, the fuel in the tank (if not completely full) will rush to the front of the tank exposing the clunk to air. If you hold it in this position for any period of time, the engine will likely start to sputter and die. This doesn't happen in flight because when nose down the inertia of accelaration pushes the fuel to the back of the tank, where the clunk will still be able to draw fuel. Think of swinging a bucket of water in a complete loop above your head.