with 2 bucket system...1st bucket has the soapy water, the 2nd bucket just plain water. you put your washing mitt in the 1st bucket.. wash your car, after the mitt is all dirty, dunk it in the 2nd bucket, rinse it throughly before putting it into the 1st bucket..
reason is because this will keep the dirt and grit out of the soapy water which could damage your car (e.g. if you use 1 bucket, that bucket will become dirty with dirt like sand.. imagine you rinse in dirty water, the sand get stuck on the mitt and you use that to wash your car.. sand+rubbing on paint = scratch). another advantage is that the suds in the soapy water will last longer as the car wash shampoo doesnt need to react with the dirt you put back into it if you're using 1 bucket