Why two buckets?
One holds soapy water, the other is your rinse bucket. This prevents dirty water from saturating your paint, reducing smears, swirls and scratches.
How do the dirt guards work?
The dirt guards have holes to allow dirt to sink to the base of the bucket, and fins to prevent the dirt circulating freely in the upper area of the bucket. When you grab fresh wash solution, it should have less dirt in than would otherwise be the case. You can use them in wash and rinse buckets - the less dirt near a mitt at any time, the better.
Suitable for wheels too?
Absolutely. Just get fresh wash solution and refresh the rinse bucket. Wheels are particularly dirty. Or do the wheels last at the end of the wash stage if you don't want to refill the buckets. Whilst there's more chance of swirls, they are less likely to be visible on wheels.
How do I clean the buckets after use?
Just take the dirt guards out and rinse out any grit particles at the base of each bucket. Don't stack them until they are dry or they may be difficult to unstick.