According to the developer’s site this is what the app is all about:
Sick of not having anyone to jam or practice with on popular Jazz Standards? Sick of those play-a-long CDs that you have to pan to the left or right? Wish you had more control to cut the bass OR drums OR piano OR lead?
Jam will change the way you practice your music. Play-a-long with real musicians – controlling how loud each musician is, or whether they are playing at all.
There are four different tracks. If you don’t want the piano, simply turn it off. If you only want the drums, no problem! If the lead is too loud, you can turn it down or off without affecting the volume of the other instruments. You can change everything live, while the track plays, without ever worrying about them staying in sync!
These are REAL recorded musicians. No computer-generated backings which don’t help your feel. It is so useful if you are a musician but even non-musicians can enjoy live mixing jazz tracks.
Interesting idea, and I think I can say that no one’s done something like this before.