Locked Axis

– This parameter let you fix one of the three object axis. The object will be able to rotate only around this fixed axis.

– Look at this die:

Die with Axis

– We want the Z axis of this die point to the Empty over it:
– X Axis fixed: the die rotates only when moving along the Y Axis (video on the rigth). Observe the red X axis of the die, its direction never changes!
– Y Axis fixed: the die rotates only when moving along the X Axis (video on the left). Observe the green Y axis of the die, its direction never changes!
– In both cases the die flips (by a 180┬║ rotation around the fixed axis) when moving vertically and passing through the Target.

Blender Track To Axis