I think it's easiest to explain what reflection does with a little gif.
Each head bone is parented to a bone, where the scale Y axis is animated from +1 to -1 and back again.
On the left both scale
and reflection
is enabled on the head bone.
In the middle, only reflection
is enabled on the head bone.
On the right, neither scale
or reflection
is enabled on the head bone.
Basically, as you can see, when the parent bone has a negative scale on X, reflection will make the child bone flip even if inherit scale
is disabled.
Note: you can only disable reflection
if inherit scale
is off.