Hi,
Yes, it reduces suction but also it does something else that just reducing the suction on the vac does not. It creates an additional inlet for air and keeps the cleaning tools from getting sucked down tight to the object and then just closing off. For example if I vac the rubber mat in my shop even on a lower vac setting the cleaning head tends to "stick" to the mat. Slide the green thingy open a couple notches and no more sticking.
Seth