A diff alows the outside wheel, of a vehicle that is turning, to turn backwards. Thusly the outside wheel turns a lesser distance. Giving you a smaller turning radius.

With that said......locking the diff doesn't allow the outside wheel to turn backwards. A lot off offroad/rock climbing people lock there diff for when one wheel comes off the ground, not power is giving to that wheel, and all the power of the motor is givin to the wheel that is making contact. Giving you more umph to get over the obsticle.

Hope this helps,
Haydn