What an idiot! Apart from the obvious dangers involved, he simply doesn't understand the fundamental principles or even the operation of a miter saw: that it BOTH mitres AND bevels simultaneously to eliminate the need for this type of lunacy.
There's free access miter saw calculators available on the interweb, and each & every smart 'phone too, that will allow the swift, geometrically accurate adjustments for an infinite number of miter and bevel angles whilst the mouldings are safely held flat against the turntable.