Article 21 and the Doctrine of Mainspring of Government

Ye section sapne mein aata hai ab! I've been trying to wrap my head around the Doctrine of Mainspring of Government and its interaction with Article 21. Section 13 of the Constitution says that all executive power of the Union is vested in the President, but Article 21 says that no person can be deprived of their life and liberty. But then there's the Doctrine of Mainspring which supposedly says that if the legislative is the mainspring, the President's powers can't be directly challenged. Or can they? I've been reading through State of W.B. v. Anwar Ali Sarkar and Keshavananda Bharti, but I still don't get how these three fit together. Is it just me or is this whole section a jumbled mess?