Decoupling theorem in top productions/decays revisited -- To what extent can we understand it visually?

I revisit here the decoupling theorem in top-quark productions/decays, which states that the angular distribution of any final-particle produced in those processes does not depend on any possible nonstandard top-quark decay interactions at their leading order when certain conditions are satisfied. Towards a simple, intuitive and visual understanding of this theorem, I will study to what extent we could explain why such a theorem holds without relying on any specific/detailed calculations.