When it's at wither extreme, you hear just that channel. Cue Level dial - this controls the volume of music the DJ hears through his headphones only. Cue Mix dial - this allows the DJ to mix between channel one and two through his headphones only. With this, you can preview and synchronize two records before the audience hears them. DJ headphones use inch plugs.
Lights flash to the beats, and flash at the same time if the sounds are in sync.
Though this is a handy visual guide for beatmatching, good DJs don't have to rely solely on this feature. This is the most accurate method of beatmatching, of getting the two records in sync, but good DJs still rely on their ears to match up the two channels. Channel Faders - these fade the volume of each channel. Talkover button - this button instantly minimizes the sound output so the DJ can talk over the music to make announcements and such. Channel Selectors - this allows you to switch between different input sources as well as, for example, switching from a CD player to a turntable on channel two.