If your talking about while you're in the air, it has to do with the gyro effect of the rear wheel spinning. if you are in the air and your front wheel is too low revving the bike spins the rear faster and raises the front in the air (usually done when you figure that if you land with the front that low you will crash so you panic and rev, hence the name) the brake tap is the opposite effect, to lower the front end, be careful, on some mx bikes this can lead to a stall in the air (potentially painful) and the same thing can be accomplished by a slight roll off on the lip (once you have the distance made) or you can just push the front end down, takes more energy but less chance of stalling.
I have just finished a having a big ramp put on my 46ft table top. The bike is pitched up higher than on any of my other jumps, probably like 8-12 ft high from the table. I never noticed the panic rev much but with jump it is really evident. I don't like landing with the power off or slightly on so I RC it and try to have almost max power to the ground. With this bigger jump I found the bike changing enough to correct a nose-down attitude of about 7 degrees. Maybe it is the higher more weightless one gets the more the rev/brake effect has.