Irlandes,
The downwash velocity is measured relative to the free stream ie, the air mass that the helo is operating in. The air motion is that which the rotor induces on the air in order to generate the lift. If the aircraft is rising or descending, the downwash velocity is a constant relative to the air around. That is why the VRS state can occur, the downwash velocity matches the descent rate to start the VRS process.
Imagine a swimmer in a body of water, stroking with his arms. As he strokes, he is thrusting packages of water backwards to pull himself forward. We can estimate the drag of the swimmer if we could measure the backward flowing stream of water he leaves behind. The momentum change on the water is required to create the forward propulsive force.
Nick