Here is a clarification from one of our developer on the behavior of Maya Paint Skin Weights Smooth brush. We have all experience its odd behavior, hopefully this will shed some light onto it:
Smooth flood in Artisan spreads the weighting of the current joint outward to more of the surface (i.e. it spreads the white to the black parts of the rest of the surface, with enough clicks gradually making it uniformly gray). If you have maintain max influences on with a small max, this can wreak some havoc with your weighting. Here is sort of a trivial example but hopefully it is explains how it works to be useful:
Let's say you have a point between the elbow and the shoulder -- so it is weighted 0.5 to the shoulder and 0.5 to the elbow. Now, in Artisan if you smooth flood the weight for the hand joint 2-3 times, Artisan will gradually distribute the weight outwards and will paint a small weight, let's say 0.02 for this point to your hand region. If you have "Maintain Max Influences" on and a max influence of 2, then Maya has to remove the weight from the shoulder in order to add that 0.02 that has been requested for the hand. That means that the points near the elbow suddenly are weighted 0.98 to the elbow and 0.02 to the wrist and nothing to the shoulder. (With maintain max influences off, it would just take 0.02 off the shoulder.)
So while Artisan smoothed out the weights for the hand it unsmoothed out the weights for the region near elbow. From then on, maintain max influences will maintain the existing 2 influences during painting, so to fix it, you would need to start adding weight to the shoulder again, to cause it to lose the weight on the hand.
Thank you Barb!