It should be obvious.
When the diff is locked, the rear wheels rotate at the same speed, so if you do a full-lock turn with it locked, one wheel (normally the inside wheel) will skid and hop.
If you do this, make sure you do it on wet grass, soil or gravel, because on asphalt you create huge forces trying to twist the two shafts at different speeds (wind-up) and something might give.
When the diff is locked, the rear wheels rotate at the same speed, so if you do a full-lock turn with it locked, one wheel (normally the inside wheel) will skid and hop.
If you do this, make sure you do it on wet grass, soil or gravel, because on asphalt you create huge forces trying to twist the two shafts at different speeds (wind-up) and something might give.