blip your throttle dude as you downshift...
takes abit of practice, but essentially you're locking up your rear wheel cause you're not matching the engine speed to the speed that your drivetrain and wheel is going. When you pull in the clutch to downgear, obviously your engine speed will drop right? but your wheel and gears are still going at whatever speed they already are; so when you click down a gear and re-engage the clutch, you're immediately putting the engine back into the higher RPM range that your drivetrain and wheel is going at. in a manual car you'd feel it as engine braking..in a bike if severe enough, it'd cause your wheel to lock up..the remedy? below VV
to do so, as you downshift you pull your clutch in, click down a gear, then before you release the clutch, quickly rev the engine (doesn't need to be all out, just a quick flick of the wrist hence the term 'blip') This will decrease the chance of your rear locking up because you're revving your engine to an RPM range that is as close as possible to your actual drivetrain and wheel speed.
once you get good at that you can learn to downgear/blip/brake at the same time

plus it has the added bonus of sounding leet
anyone feel free to correct me if I'm off, but im 99.99% sure