From Gene at Audioholics...
"NOTE: Some Receivers have an impedance selector switch. DON'T USE THEM! The manufacturer puts them there for UL approvals as well as easing consumer concerns about driving low impedance loads. These switches step down voltage feed to the power sections which will limit dynamics and overall fidelity. Keep the switch set for 8 ohms regardless of the impedance of your speakers and ensure proper ventilation of the Receiver.
Update: 09/06/05
Upon further investigation on this topic we have discovered that in most cases the switch is doing nothing more than reducing the rail voltage. Please check out our measurements on page 2 of the Yamaha R-XV4600 review for more information on this.
As you can see the measurement differences between the "low" setting (less than 8 ohms) and the "High" setting (8 ohms or more). This is the reason I usually recommend keeping this switch in its default "High" setting and using common sense when mating a receiver with inefficient 4 ohm speakers in large rooms.
All the "Low" setting of the switch is doing is stepping the rail voltage down so when UL tests the amp at a specified distortion level, the amp will achieve that distortion level sooner since it runs out of headroom more quickly than it would in the "High" (8 ohm or more) setting. This in turn generates less heat since the amp isn't driven as hard. You really aren't buying any protection for driving low impedance loads as you actually risk clipping the amplifier more since it can run out of headroom more easily. The switch is there more for certification purposes. The reason you don't see this switch on separate amps is twofold:
They typically have more heat sink area, and bigger power supplies and can better manage the heat
They aren't UL certified and don't have to meet the requirement. "
