back pressure, it keeps the fuel in the cylinder for longer.
think of of it as expanding gas, only so much can fit thru the stinger at a time, so alot of the unburned fuel gets back into the cylinder. it also seems to make the pipe longer.
im not a pro at this tho, im sure somone else here has a lot more insightful input, i just know less free flowing pipe = more torque and a more free flowing pipe means a better top speed aka rpms.
~SABAT!