The Chirp skratch was invented by Jazzy Jeff and is widely used by many Dj's worldwide.
1) First, find the cut in point on your fader.
2) Start with the fader turned on.
3) Play the sample forward, just as you find your sample cut in, cut the fader off.
4) Drag your sample backwards and quickly turn the fader on catching the start of the sample in reverse.
5) Keep repeating this process over and over, you will notice once you get the hang of the technic that your chirping speed will increase and the consistantisity will become greater.
So remember this;
1) Fader on, sample forward.
2) Cut fader off after sample has begun.
3) Drag sample backwards.
4) Cut fader in catching beginning of your sample.