When scratching a record you can mute the sound in either direction by holding down the Ctrl or Shift keys.
Ctrl will mute back spins.
Shift will mute forward spins.
When you let go of the record the Ctrl muting will stop even if you have the button held down. However Shift will continue the muting until you let go of the button. (If you are playing a song in the backwards direction mode the opposite is true).
How to Scratch
Scratching - Left button release momentum
Applying Scratch samples