I'm assuming you are talking about matching engine speed to the proper gear speed. You want them to be as close to the same if powershifting around a road course.
Meaning, if you were in fourth (or fifth or sixth) and are braking for a corner, now need to downshift to third, you should try to have the engine spinning at an rpm equal to the rpm your car will be at once you complete the shift. Spin to fast, and you can break loose the tires. Spin to slow, you can temporarily lock up the rear tires trying to catch the engine speed up to tire speed. The latter is especially bad when driving very aggressively at high speeds.
The idea is to downshift going into a corner so that you will be in the proper gear to accelerate hard out of the turn. If you don't rev-match (heel & toe) you will upset the balance of the car, and that leads to trouble.
You can practice doing this using on/off ramps (if traffic conditions allow) to get the feel for it. PFYC.com has great split screen video with an in-car camera view that you can watch also. You can see the driver using heel and toe around various parts of the tracks. Also, LG just posted a video of their car going around the road course and if you listen you can hear Lou rev-matching into corners.
And although Heal and Toe is the prefered method, you don't have to use that style of rev-matching.
On the street, I rev-match like you would if you were double clutching an old truck. It seems to work pretty well. My '02 Z06 has over 85,000-miles of roadracing, autocross, dragstrip and rush hour city driving with no tranny problems.
Easiest way to learn is to start with a straight piece of road and cruising along at 2K RPM in 5th.
If you wanted to drop the hammer on someone, you would want 3rd gear. To get 3rd you shift to neutral and let up the clutch, then rev the engine to bring the transmission's input shaft up to a speed higher than it will need to mesh with the driven gear. Then step on the clutch and shift into 3rd.
If done smoothly, there is no drop or jump in engine RPM as the clutch is released.
Heal -Toe involves using the right foot to apply the brake and blip the throttle while downshifting. The accepted practice with vettes is to use the brakes to do all the stopping on corner entry while still going straight, and then match up the right gear and drive through the corner on the throttle. At least as I've read, and find works well for me.
Not having the rev's matched (too low) will cause the rear tires to drag and possibly upset the balance.
I use the ball of my foot on the brakes, and the outside of my heel to work the gas.
I hope that helps out, good luck getting it figured out cause I'm still learning.