Drag clicking relies on a physical acoustic principle known as stick-slip friction — the exact same mechanism that makes a violin string sing when a bow slides across it.
When you glide your dry finger along a textured mouse button at a specific angle and downward pressure, microscopic friction creates rapid alternating cycles of sticking (gripping the surface) and slipping (releasing forward). Each bounce drives the mouse switch down with incredible speed, registering 20 to 50+ clicks in under half a second.