[Dave at Pelican Parts]

The complete explanation is long an fairly ugly...

The short version is that the exhaust gases travel in pulses (the valve opens, the contents of the cylinder get pushed out, the valve closes). "Behind" each pulse is a little vacuum. If you merge the exhaust pipes together the right way, you can actually use those little vacuums to suck out more of the exhaust gases from the next exhaust valve to open up. This more effectively scavenges all the burned, used-up air out, and creates a slightly lower-pressure area in the cylinder which helps pack in a tiny bit more air the next time the intake valve opens.

There are actually about three or four effects that go on, but the net product is that they are used to scavenge more of the exhaust gases out.

Not precisely less back-pressure, but I guess that idea will do as a first approximation.

--DD