Iv'e done this for over 40 years , started out doing it on 2-strokes.That's old school. I put a bolt with a washer welded to the end through the bottom of the pipes on my stock exhaust after a partial gut back in 2008. Nuts inside & out. The washers could be rotated 90 degrees to change the back pressure. Guys have done this forever to (try to) tweak their exhaust.
AR cones , anti reversion cones.The 109 calls theirs the SET valve, for Suzuki Exhaust tuning. Same thing as the EXUP valve on the Yamaha.
I don't know if they still sell them or not, but they used to have a thing called "Torque Cones" that accomplished what your friends did to their headers. It was shaped like a short funnel and dropped into the top of the head pipe where it bolts to the cylinder heads.