If viewing DAMN banks, the 'Set Masks' button should be grayed out and unusable.. you may not change the values of DAMN banks, as they are defined by the database! However, if you are looking at DARN banks, this button will be available.
Quite simply, the mask value can be changed by either typing in a new value in the numerical field, or by clicking on the toggle-buttons. Pressing the 'Set Masks' button sends the new value to DMM by calling dmm_set_mask.
The new value is NOT set until this 'Set Masks' button is pushed!
Pressing the 'Get Masks' button redisplays the current 'true' values of the masks at any time.