Basically you reroute the cylinder head breather hoses "outside of the motor" instead of directly back to air intake as stock. For my bike the breather reroute did wonders; the bike runs much smoother since.

My take on the top 5 necessary mods (not in particular order):
- Breather reroute
- Right side scoop
- ECM tune
- Tail chop
- Air filter

(bubbling under: shorty levers & bar ends)

Superman; breather reroute is maybe not absolutely needed, but highly recommended. Some say it won't help at all, some (like me) would not change back once tried. It is so easy and cheap to do that you won't lose anything if you try it. You can always revert to stock if you don't like it.