How do you think this was shot? As the shot locks into position where a "mirror" is facing directly at us, there should be a camera and film crew shown in the "reflection", but no such reflection is present.
It could be that there is no mirror but a mirrored stage behind to create the reflection effect. But at a cost of having to match the movements and timing perfectly in the reflection, bear in mind back then, in order to review footage, the film needed to be developed first then to projected.
Watch again the timing and pyhics of the stocking coming off the foot is perfectly matched in the reflection, which arguably is impossible to replicate perfectly.