Rev-up or Rev matching is a method of making a smoother shift:
If you use Double declutch, you are probably driving an old car, a heavy truck or trying to complete a J-Turn without locking the front tires. What you need to do is to de-clutch, shift into neutral, release the clutch and than rev-up the engine with a little dab on the gas, de-clutch yet again and select the next gear.
If you use a simple shift (very rare) all you need to do is to dab the accelerator while the clutch is depressed, just before you turn into the corner. Now, re-engage the clutch.
Note that unless the car has an automated gearbox, like DSG, trying to do a clutchless shift in high-revs will not help you go faster and can cause gearbox damage.