Using the Crossfader
When you load music into each of the decks and play them, moving the crossfader from side to side fades in between the two pieces of music as they play. When the crossfader is set completely to the left you will only hear the music playing in Deck A. When you set the crossfader completely to the right you will only hear the music playing in Deck B.
The idea is that as a track comes close to finishing in deck A, you begin to fade in a new track from deck B. Then while deck B is playing, you keep the crossfader fully to the right. This allows you to load a new piece of music into deck A so when deck B starts to finish you can start the music in deck A and then slowly push the crossfader away from deck B to nicely bring in the new music from Deck A.
The crossfader can also be manually incremented (and decremented) or set to full left and right using hotkeys.
Crossfade Buttons
The crossfade buttons sit on the left and right of the crossfader. These perform a smooth auto crossfade from one deck to the other. The time it takes to crossfade can be set in the Options under the General tab.
Crossfading is automatically handled when in Auto Play mode to mix your music for you.