Downshifting without using the clutch involves a rapid “blip” of the throttle. That throttle blip reverses the deceleration force on the transmission. In that brief moment of transition from deceleration to acceleration, the load on the gearbox is relaxed, which allows for a downshift the same way the clutch does. Plus, as the gear is being selected, there is another brief window of time when none are engaged. In that moment, the throttle blip free-revs the engine, allowing a perfect downshift with revs matched to the new gear.