With the joint of a mill to nod or swivel, that creates a point of rotation. Lengthening distance [hypotenuse] from indicator point to that center, like a sine bar, angle doesn't change but the side opposite 'expands' so indicator will detect out of perpendicularity.
I'm describing as if a right angle, but the condition is more truly scalene cone until quill is at a right angle, thereby an isosceles.