Rev matching is a technique used to change gears smoothly and with minimal disruption to the car's balance. It's also commonly known as blipping the throttle. Specifically, the aim is to match the speed of the engine with the speed of the transmission. It's mostly used when changing down a gear, rev matching makes a huge difference in both the lifetime of your engine and your lap times.
Changing down to a lower gear without properly adjusting your speed can cause excess strain on your engine, clutch and gears. By matching the revs, your car won't jerk forwards due to engine braking which you may have experienced whilst driving in the past.
Rev matching is also used in the process of heel-and-toe, a technique commonly used by racing drivers who will use the technique on the approach to a corner. This is because when you're pushing your car to the limit, a sudden transfer of the position of the cars weight can have dangerous consequences, such as losing grip and spinning out.
Let's use the example of changing down from third gear at 3000 RPM into second gear. If you simply put the clutch in, move your gear stick into second and release the clutch, you'll feel the engine harshly lurch forward and see your RPM jump up to around 5500 RPM. To match the revs, you need to instead put the clutch in and immediately press the throttle and increase the revs towards 5500, then release the clutch.
Gear ratios are specific to each different car, so you'll have to understand your own cars gear ratios to do this properly.
If you still enjoy driving a manual but are a little bit rusty when it comes to matching the throttle with your gears, some cars now come with an automated rev matching system. In a car equipped with this feature, your car will automatically blip the throttle every time you change down a gear.
If you change down into the wrong gear by accident, you'll be safe from some damage to your internals though motorists around you may think you're acting like a boy racer.
There aren't many cars to choose from as manufacturers are starting to opt for an automatic transmission on their performance models. Here are some of the models available with automatic rev matching:
Yes, it does use more fuel due to the sharp use of the accelerator pedal, however the extra fuel usage is very minimal.
If you're using rev matching with the intention to slowly engine brake to a complete halt, then yes - but that's more to do with engine braking than the act of actually rev matching. Generally though, no, it isn't bad to rev match as it helps to reduce clutch and engine wear, but only a small percentage of motorists will actually drive this way and their car will still last for a similar amount of time.