Airing down is a key part of any off road trip. In the world of 4wding, what determines if you make it or get stuck is often your tyre pressures. It simply means the difference between maintaining traction or spinning your wheels. If you're not running your tyres at a low enough pressure, no air locker, low range gear or amount of power is going to help you conquer that section of track.
Lowering tyre pressures to the appropriate amount for any terrain is the single best modifications to your vehicle you can make. It's also the cheapest! With this logic, there are products on the market that let you deflate your tyres with ease when you're ready to hit the dirt.
The gauge type tyre deflators are seemingly awkward to use at first. They have 3 knobs, turn the middle knob clockwise to attach the deflator to the tyre stem, then turn the outer knob to remove the valve core. Pull the stem out to remove the valve core and finally pull the fat inner knob to release air pressure.
The proceedure removes the valve core and keeps it trapped to ensure it won't go flying when it is removed. You can then keep an eye on the gauge until you've got the right air pressure. You then reverse the whole proceedure to re-install the valve core into the valve stem.
Pros: They are an inexpensive product that gets the job done.
Cons: You need to walk around to each individual tyre and repeat the proceedure to air down.
Automatic tyre deflators are an adjustable type automatic shut off device that lets you screw on 4 deflators and relax while the tyres deflate themselves to your set pressure. Initial setup is simple and effective, using an already deflated tyre at the pressure you wish to set the deflators to, configure the tyre deflator to it's highest set pressure, then adjust down until the valve opens up. Once you reach this point, use the lock ring to keep the deflator at this set pressure.
Once the deflators are setup, you can screw them onto the valve stems and forget them. It is not recommended to drive the entire track with the deflators on, however they are suitable to drive to a campsite or first stop with them still on the tyre valves.
Pros: Very simple to use, automatic shut off. Range from cheap to expensive.
Cons: Requires initial setup.