Mopping is done typically after the initial rub as been set on the meat by the heat in the smoker.
It is a gentle process, so as not to disturb the existing rub/bark/crust.
Mopping sauce is a thin sauce. I can be a dluted BBQ sauce, but as mentioned watch the sugar.
My mops typically contain paprika, onion, garlic, cider vinegar and bourbon. Other ingredients depending are beer, tequlia, worchester, Franks, rosemary, sage, etc.
On Edit: Never mop while diving. You will break up the rub.