Matching engine speed with the rear tire speed. You could also install a slipper clutch, if there is one for your bike, that takes care of rev matching for you.
If you’ve ever driven a manual car and downshifted and just let out the clutch, the car will jerk. In cars there’s a technique called double clutching which revs the engine rpm to match were it was at in the higher gear eliminating that jerk. Easing out the clutch and blipping the throttle, more or less, does the same thing on a motorcycle. Just doing straight release the clutch on a downshift will upset the chasis, like the jerking in a car, which you pretty much don’t want to do.
edit: I should describe blipping the throttle. You slow down (engine braking or using your brakes or both), pull in the clutch lever, open the throttle with the clutch lever still in, down shift and release the clutch. I do it ass backwards myself though… I pull in the clutch, downshift, slowdown, throttle, and then release the clutch.
I bet fotobits could describe this blipping thing a whole lot better…
edit2: just re-reading this and it’s still not complete. open the throttle with the clutch lever still in == quick rev or 2 (the blip). downshift and release the clutch should be downshift and release the clutch while smoothly rolling on the throttle. and i still do it ass backwards but with the clarifications above.
If there’s anything more important than my ego
around, I want it caught and shot now…