This is a really nifty addition to the Weber Kettle and it can also work on the Weber Smokey Mountain and a variety of other devices with adapters.
The PartyQ controls the temp in the cooker by controlling the oxygen that reaches the coals. PartyQ is a battery operated fan connected to a thermometer probe that you clip to the grate near the meat. You have to drill a hole near the bottom of your kettle for the blower to mount. Then close the intakes, open the top vent, set the target temp, and it shuts off the air supply when the temp gets above it, and when it drops below, the fan supplies air to the coals.
We know one Slow 'N Sear owner who's reported maintaining exactly 225°F all 25 times he checked the temps throughout the cook, for 12 hours, using this combination with no refilling the charcoal basket. You can get the same cook times without this accessory, the temps just won't be quite as rock steady.