they increase power through a certain rpm range, usually about 2000 rpm wide. And it prevents reving past that range so you have to adjust the header length so that the pipe gives you the range you want. On many pipes also they cause a dip in power for 1000 or more rpm below the powerband of the pipe. They are ideal for engines with gears. oh, and they are loud as hell but you can modify them to be quieter by making the stinger exit from the belly of the pipe instead of at the end.
