It sits in the bottom of a wash bucket and keeps the grit you rinse off your mitt separated at the base, so it is not swirling back into the water you are about to put on your paint. That is the entire mechanism, and it works: agitating the mitt against the guard knocks dirt out of the fibres and the raised platform stops you dragging it back up. It is the cheapest scratch-prevention item in the wash bay.

Keep one bucket with clean shampoo solution and one with plain rinse water, both fitted with grit guards. After washing a panel, rinse and agitate the mitt against the guard in the rinse bucket before it goes anywhere near the soap bucket, because the mitt still carries significant dirt at that point. Wash top to bottom, leaving the filthiest lower panels last, and change the rinse water if it turns visibly dirty.

A thorough pre-wash that leaves the paint 80 to 90 percent clean before you touch it does most of the work, and after that a single bucket with a grit guard is generally sufficient. The two-bucket method exists to manage the grit a contact wash picks up, so the less grit there is to begin with, the less it matters. If you skip the pre-wash, use two buckets, since that is when the risk is highest.

A 20 litre bucket is the practical standard: enough water that the dilution stays effective through a whole car, and wide enough for a grit guard to sit flat with room to agitate a mitt. Smaller buckets get dirty too quickly and cramp the mitt. A Snappy Grip style ergonomic handle is worth adding if you carry full buckets any distance, since a wire handle on 20 litres is uncomfortable and hard to control.