It's not explained in the description but playing the musical instruments on beats gives you combo, which I assume makes instruments hit harder. (and I love it!)
The beats of the instruments are kinda out of sync with the music, though. I'm not sure if this is because of the input lag or game itself lag.
Also, I feel like there's not enough juice in each hit. When the player or the enemies get hurt, they just groan and have a stat change. They don't even twitch, glow red or emit particles, which makes it kinda hard to tell if you hit them or they hit you. The floor is also kinda uneven and the player often get stuck while walking around. I do believe that all of these are just small problems that are easy to fix, though.