The scale deform mode includes rotation by default, so when when scaling
down a part of the models it becomes harder to control as the effect of
the rotation less predictable (similar to using trackball rotation in a
very small radius). This locks the rotation of the segment, so parts of
the model can be scaled down in a more predictable way.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D8465
9ea77f5232