how fast your clutches shift back down into a lower ratio.
When you accelerate your primary closes, pushes the belt up which in turn pulls the belt down into the secondary (opening it) this inturn puts the sled into a higher gear like sprokets (distance around) when you let off the throttle the oposite happens which puts the clutches into a lower gear (backshift, or like downshifting), the rate of how fast each of these actions happen is rate of upshift and backshift.