This small lever on the bottom of the mixer is every old school DJ’s best friend. Basically, the crossfader is simply the lever that balances the mix level of two tracks in the line out. The more it goes on the left, the more the left track is privileged (and viceversa).
Nowadays, crossfader lost part of its importance, even if it is still used a lot. When “we” (meant as DJs) used to play with vinyls, the crossfader was essential for the mix. With the arrival of the CDJs during the 90s that lever was partially substituted with the track volume levers, that allow a deeper and independent control of the mix level for each playing track. With the latest technologies made of pendrives and mp3s and controllers (not going into how much I don’t like ’em) crossfader became a vestigial organ of the mixer, something still present because of its usage but actually not used exception made for hip hop DJs.
Crossfader is what allowed and allows DJs to perform heavy scratch sessions. With one hand you hold the vinyl or the spinpad, with the other you hold the crossfader. Usually, when you go for a scratch you also move the crossfader to give a “wave” effect to the output. More details of this can be found in the video I’m linking here!
That’s all folks, less sync more scratch!