With VMC testing. Picture this for a second, a multi-engine the airplane is at full takeoff thrust because it's just taken off or it's in a go-around, and suddenly an engine fails. Now you're faced with full thrust on one side and windmilling drag on the other. Can you maintain directional control? Can you fly safely? The answer depends on airspeed. The slowest speed you can be to maintain directional control of an airplane in this condition is called the VMC or minimum control speed.
The regulations define a couple different flavors of minimum control speed VMC-A is the minimum control speed, while airborne VMC-G is the minimum control speed on the ground. That would be for an engine failure as you're accelerating to takeoff speed. We have to test these during certification efforts because we have to be absolutely sure that the pilot can maintain control and continue safe flight to landing in the event of an engine failure no matter what the thrust condition was beforehand.
