Why does the method of substituting various trig functions in the angle of an inverse trig function to simplify/differentiate it work?

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I have learnt that to simplify or to make differentiation of inverse trigonometric functions easier, you can simply substitute trigonometric functions in the angle for literally any sort of variable you want, so that you can express the angle as a trig identity, apply it, and thus simplify the function. Sure, it is effective. But why does it work? There must be some limitation on the type of variables you substitute, which I haven't seen yet. And, won't the simplification work for just that specific substitution only? How can it be generalised?