We run common vacuum cleaners for vacuum panel lifter movers all day long. when a vac is restricted or even plugged it draws less current than when open. More air more current, less air less current. Blowers are designed with applied HP in mind. try it with the wifes vacuum cleaner. turn it on and plug the end. what do you hear? the rpms go up. an indication that the motor is under less load. caution! most of these motors are air over for cooling so do not do this for an extended period of time or the motor will overheat from lack of cooling not increased load. Fridge is right when he says that an unloaded motor will not run at fully synchronous speed because of slip and that it will run without harm all day. He was missing where the LOAD on the motor increases as the volume of air allowed to pass through the Blower increases. Fred missed it with where the restriction has to be. it is total air movement limitation whether it is gaged before or after the fan makes no difference. there are thousands of examples of both dirty side and clean side blowers in the wood working industry out there each has its advantages for the user. Rick pegged the energy loss part of the equation. a bathroom fan averages 100 cfm.