Anti-reversion cone in the header pipe. Basically a funnel stuck into the engine end of the pipe to give back pulses something to bounce off of.
I didn't use one in mine. Ended up running a JC Whitney 12" universal glasspack, gutted since the glass packing disappears quick fast and in a hurry. Had to weld a stick stopper baffle in the pipe to be legal out here (if a night stick fits fully in the pipe it is illegal). Dropped in a few dollar store pot scrubbers to mellow it out. A bit too quiet right now, but, the scrubbies won't last forever...
Seriously, straight pipes look good and sound mean but you are looking at some serious tickets if you have neighbors, live in town, ride in town, etc. At least put a 3" long baffle in it to calm things down a touch. You'll have your sound, some back pressure, and Kojak can't rape your pipe.