I suspect that the reference is to Vmu - Minimum Unstick speed. Done as part of normal certification flight testing to ascertain the minimum speed at which the aircraft can be forced to 'break ground'. It's a part of the [rocess of deriving Vr.
Some pretty impressive testing videos from Boeing's flight tests if you can get them, the aircraft with a lump of wood attached to the rear under-fuselage for the inevitable (deliberate?) tail strike, the wood being in abundant supply from the forest just across the road from Boeing.
Regards,
Old Smokey