I never knew there was turbulance...all that is happening is the gap is closed, how would that create back pressure when it's just flowing like it would without the cutout on?

 

turbulance due to the notch where is will be closed, for example a straw, a brand new straw is perfectly smooth but cut a notch in the middle to open up and when you try and close it will no longer be smooth, haha i think this is a really brutal example but you somewhat get the picture

 

that and the gases are flowing into a 10 inch or whatever long side tube... to which there is no escape... so they try to make their way back...

but the effects are, im sure, minimal.

whenever you wanna give someone a quick drag on the street... just hit the switch and open it. if its electric that is =]