So I learned of this option today in Zbrush and thought I would share here so others that have had the same issues will know how to fix them in one very quick easy button click!
Let me set the stage for you, your sculpting away say on a inner ear, your loving your sculpt! then you rotate the camera to find you have destroyed the back side of the ear because you forgot to set up polygroups and hide sections or forgot to mask the back. Seeing this you scream out WHY!!!!!!!!!!!!!!! (insert questionable language here of your choosing) then set about undoing or trying to fix the now destroyed area or just giving up and opening a old save and doing it again the "right" way...
Well no longer
Simply just go to "Brush" and down to "Auto Masking" then click "BackFaceMask" never again have this issue!!! to make sure your "Shift option" "Smooth" also has this applied hold shift well in the menu and the button will become deselected click it again well holding "shift" and now both will have this setting applied!!! (Note this is a per brush option...)
Hope this helps, it sure made my day learning of this button!!