DIG is additional amperage when the load voltage drops below a set value. Miller's machines adds the additional amperage around 19-20 volts. When normal arc lengths are used the set amperage will be the only amperage while welding. At normal arc lengths the load voltage will be 25 to 40 volts. Therefore it is well above the 19-20 trigger volts. Set the main amperage control for the penetration and arc characteristics with the DIG at 0, then fine tune adjust the arc with amount of DIG that you like. Have someone watch the meters and crowd the electrode arc length, push it in and see what the voltage drops to.
"Amperage controls the penetration, voltage controls the height and width of the weld deposit". Cut & Etch. A good test is worth a thousand opinions.