The 509 anti-fog fan, stops fog dead in it's tracks. If you are a heavy sweater or just want an extra level of fog stopping protection, the 509 anti-fog fan is for you. This is a removable fan module that installs directly into your existing pair of 509 goggles. In fact this 509 version was specifically re-engineered to work with both 509 Sinister & Aviator goggles. No other fan product on the market is directly compatible with both 509 goggle models. In addition, it's compatible with virtually every other goggle brand on the market. Once installed in your goggles, the fan module has 3 settings. Off, On, and Auto. The auto setting automatically detects fog and turns on the fan only when needed. Just set it on auto and let the module do all the work. The fan works by pulling moisture from inside the goggle and pushing it out the top of your goggles. When the moisture level drops to an acceptable level, the fan will automatically turn off. The fan is powered by one AAA battery, which is included.