Hi how air proof or airtight is the throttle valve in the system, on my bike I am able to pressing air through with throttle close, and can't tell any difference if it is close or open
so are you saying engine off pulling the clutch in you can here air coming through the throttle bodies? if so that's corrects..
.when you back off the throttle the vacuum sucks the air from the clutch pulling the diaphragm allowing clutch plates to slip slightly stopping the back wheel from locking on downshifts/over revving engine due to difference in road speed/engine speed