The other half is a mixer where you have to shift tracks between sample 1 and 2. You can go to djhero.com to learn more about it and see some of the tracks you can play in game. It looks amazing, and i think they've been cleaning it up well cause the original buttons had a HUGE variation switching between medium and hard. like hard looked almost impossible if you transitioned. Now it looks like everything is mixed well to built up difficulty.
EDIT:
To elaborate on tracklists
http://www.djhero.com/music/setlist/
O_o thats a lot of songs