In a manual tranmission car, the clutch is 'meshed' only when both sides of the clutch - the engine side and the wheel side - are running at the same speed.
I'll make up some numbers for an example: Say you're at 3rd gear, going 45 miles an hour and the engine is at 2.5k RPM. If you downshift while still going 45 miles an hour, the equivalent engine speed is now probably near 3-4k RPM for second gear at 45 MPH.
If you were to just naively disengage the clutch, switch to 2nd gear, and re-engage the clutch, all while keeping your foot off the throttle, then you've got a few things going on:
- The engine speed is going to drop while you're making the switch
- The new engine speed is going to be a lot higher than what it was before.
- The clutch is going to have to do a lot of work to bring the engine speed up to where it needs to be.
So the chain of events goes something like this:
- You're in 3rd, traveling 45 MPH and the engine speed is 2.5k RPM
- You disengage the clutch
- You start switching the gear to second gear
- The engine speed drops to 1.5k RPM in the time it took you to change gears
- The car is still traveling at 45 MPH
- 2nd gear at 45 MPH wants the engine to be 4k RPM, but the engine is currently going 1.5k RPM
- You start to engage the clutch
- The clutch "grinds" a bunch while it brings the engine up to speed
- The clutch stops grinding when its transferred enough energy to the engine to bring the engine up to 4k RPM.
- You continue on your merry way.
The biggest problem is that when downshifting, your engine speed has to go up (a lot) in order to match the change in gear ratio, and the clutch has to be the guy to do that.
Ok so with that out of the way, what is "blipping"? Blipping is known by a few other names - "burping" the throttle, rev-matching, etc, but it's all the same idea.
The idea is to manually raise the engine speed when downshifting so that the clutch has to do less work - if I disengage the clutch and give the engine some gas to manually bring it up to 4k RPM, then when I re-engage the clutch after switching to 2nd gear, then the clutch has to do very little work.
In this example, if the engine speed is already at 4k when I re-engage the clutch, then the clutch won't experience as much wear and tear, the shift can complete much sooner (important if you need to shift fast for some reason), and the ride is more comfortable because there's less of a jerk when the clutch re-engages.
All-in-all, rev-matching is a good thing for your car, but it does take some time to master. You have to develop an innate sense for what engine speed you need when you're in X gear and the car is traveling at Y speed; and then actually being able to do it in one quick procedure takes practice.
In competitive contexts, rev-matching is done, but sometimes for other reasons. Keep in mind that when downshifting, the car might jerk a little as the engine is brought up to speed - if you're a rally driver and you're trying to be very careful about when you make demands of your traction, you might try to rev-match in order to not accidentally cause your traction to break loose, for example.
Another related technique is power-shifting - applying full throttle while upshifting, ie, never releasing the throttle while trying to accelerate hard and switch gears. This is very hard on the clutch, gearbox, vehicle dynamics, but it can be helpful for your track times - during that split second while you're switching gears the engine is still making power - that power is being stored in the rising rotational energy of the engine. Then when the clutch is re-engaged, that stored energy is transferred to the vehicle, giving you a small boost. The end result is that you can accelerate just a little faster than if you let off the throttle.
Powershifting can have second-order effects too, for instance if the engine uses turbo chargers. Turbo chargers work by using the force of the exhaust gas to spin an impeller that then is used to force air into the engine. Turbo chargers are notorious for 'turbo lag' - they don't help a lot when the engine speed - and thus the volume of exhaust gas coming out - isn't that high. Turbos help make more power at higher engine speeds.
Well, if you are doing things like power shifting, then that helps keep the turbo 'spooled up' - itself turning at a high speed - so that it can be ready to help make more power for the engine.