Why people raise their hand when they knock a door?

Question

Because you can knock a door with your hand down as well and it would be more convenient?

In: Physics

Answer 1

I imagine because it’s easier to control the force and thus the volume of the knocking, especially since the door’s resonance and resistance to knocking is usually an unknown until the first knock, so that if more or less force is called for on subsequent knocks you can make quicker and finer tuned adjustment with your raised hand.