Light dimmers work by using an SCR or TRIAC to turn on after the start of each line half-cycle. It then automatically turns off at the end of each half-cycle. The percentage of each half-cycle time the SCR is on determines the average output voltage. It basically chops at the line frequency.
I tried a dimmer for my fan, which did slow the fan, but the chopping caused a buzz in the fan that was annoying. That's when I went to the series cap (which leaves the fan perfectly silent of course).