I don't ride my bike on the track, but if I did, I would brake, blip and downshift all simultaneously.
The purpose of blipping (as was already mentioned) is to match the wheel speed with the engine speed, as to not lock up the rear tire. The reason for doing this all simultaneously, is to shorten your braking zone (hense quicker lap times).
Riders that are fast on a closed racing course, brake very, very hard at the end of a fast straight, into a tight, slow corner, all the while they are blipping and downshifting at the same time.
If you're asking this question, (no disrespect) you shouldn't be taking a liter bike out on track. A riding school would also be in order.
Bogie is the man, and I should learn not to mess with him