What's throttle blipping?
When down shifting you blip the throttle while letting the clutch go, it prevents the rear wheel from locking up and really smoothes out the downshift.
"Let's say that in 3rd gear at a given speed your engine is running at 3000 rpms and in 2nd gear it's running at 4000 rpms at the same speed. If you quickly downshift and let the clutch out without attempting to match engine speed, your engine is still at 3000 rpms but the rear tire wants it to be at 4000. The rear tire can slide until it speeds the engine up to 4000 rpms to match the road speed.
However, if you blip the throttle to get the engine up to 4000 rpms before letting the clutch back out, you're now at the right speed and all will be smooth when you complete the shift."
It's definitely not always necessary to throttle blip, but you can hear in the video that he sometimes downshifts a couple gear without doing it and gets a big engine breaking noise.
This kind of thing can really lock up your rear wheel and give you a really bad surprise.
Now, if you are just strolling along in the city it's not really a big deal, but in twisty roads like that, it can ruin your day!
It is really worth doing while breaking hard before a turn to, as it allows a smoother breaking without the jerking of the gear changes.
[ame="http://www.youtube.com/watch?v=H_cPblE6my4"]How To Downshift While Rev Matching - YouTube[/ame]