The closing illusion, the buzz saw, is genuinely dangerous, and the metal cuffs used to secure Mrs. Sinclair were necessary to make sure she stayed perfectly still.
This version of the buzz saw illusion - where the saw is placed with its axle below the table and cuts through the assistant from below - is considered to be the most baffling version of the illusion, as it is obvious there is no way for the assistant to avoid the blade as it cuts through them.