A purpose-built MIDI controller for machine aided scratching. Replaces the crossfader for today's discerning turntablist.
 

When I was first interested in scratching, about 20 years ago, I couldn't afford a fancy DJ mixer or anything like that. I realized... and yes, this is an oversimplification... that when DJ's are flicking the crossfader back and forth, they are using it essentially as an on-and-off button. So I made my own "poor man's crossfader" by wiring a cable that went from the turntable to the stereo, with a pushbutton switch right in the middle. It was clicky and a little crackly and impossible to do crabs or any fast moves.
 

Over the years this device has evolved into the Interruptor, a 4-switch MIDI-controller version of the original. The roller switches allow for much more sensitive finger action and they can each be mapped to a separate track in Ableton Live, providing varying levels of automatic flares/crabs (16th notes, 32nd notes, triplets, etc.)