I mopped until a 5-6 years ago, loved the results, stopped to try something different, will probably go back to mopping.....for something different :-D
When I mop, it is usually after a couple of hours in smoker. It clearly drips off if meat is turned, moved etc., but some adheres to the surface and begins to dry, thus prompting the next mop. What adheres adds flavors to the surface, not sure how much actually makes it into meat, sort of like putting dry rub/seasoning on outside of chicken. Not much gets into the chicken, but it tastes great on the skin. My mop is roughly equal parts canola oil, butter, worcestershire, mustard, along with coarse ground pepper.
IT IS A HUGE MESS, don't do it if you are feeling a little lazy.
I grew up in southeast Texas watching my grandfather mop and just continued as I got older and began to smoke brisket. As my cooking skills improved, I found it was not necessary to keep brisket moist, but I enjoyed the additional flavor. I have his hand written recipes and the now ambiguous measurements. Stuff like.....a box of pepper, Oleo (not sure if he meant 1lb or less), bottle of mustard....