Now that they all told you how the dimmer works, lets look at the fan issue. From experience and from talking to several hood manufacturers in the past, as well as two pond pump manufacturers ... no the dimmer won't hurt the small (emphasis on small) motors. I have been told by all of them that these shaded pole motors are the world's most inefficient motors anyway and the lower voltage will not cause them to get beyond the capacity of their windings. We all know this is in theoretical conflict with George Simon (Ohm) but in real life it works.
BTW most of these dimmers are generic, low dollar dimmers that reduce the voltage ... you can verify this with your DMM.
