Mine does this too, It could be one of two things or a combo of both, check and make sure there is enough brake fluid in the system, (duh you prolly could have guessed that, so don't flame me for a stupid answer), or you if you really hammer it hard it may use so much fluid that the level in the master cylinder drops below the sensor level and triggers the light, of then when you let off it will bring the fliud back and the sensor will read normal. Even if you are a little low on fluid it would amplify the problem.