They work by varying the frequency of the AC current which in turn varies the speed of the AC motor. An AC motors speed is directly related to, and a multiple of, the AC frequency (a little less actually, because they "slip" a little under load, for example 1750 rpm, which is 1800 rpm minus a little slippage at full load). Household AC lines are 60 hz in north america. A VFD can change that to between 5 hz and 120 hz typically, so you can get from near 150 rpm to ~3500 from that same motor. You may need a special motor to go that slow and not over heat.