Special Provisions
(1) If the President is satisfied a grave emergency threatens India's (or part of it) security, from war, external aggression, or armed rebellion (even before actual occurrence, if imminent danger exists), the President may proclaim a National Emergency for all or part of India. (2)-(3) Such a Proclamation can be varied/revoked by a later one, and can only be issued after the Union Cabinet's decision is communicated to the President in writing. (4)-(6) It must be laid before Parliament and approved by both Houses (by a majority of total membership plus two-thirds of those voting) within one month, or it lapses (with special rules if the Lok Sabha is dissolved); once approved, it lasts six months, renewable indefinitely by further such resolutions. (7)-(8) The President must revoke it if the Lok Sabha disapproves it (by ordinary majority) - and if one-tenth of Lok Sabha members give notice of intent to move disapproval, a special sitting must be held within 14 days. (9) The President may issue separate Proclamations for different grounds simultaneously.
This is the constitutional basis for declaring a National Emergency - originally allowing declaration for "internal disturbance" too, but the 44th Amendment (1978) replaced that with the narrower "armed rebellion" specifically in response to the 1975-77 Emergency's widely-criticised misuse, along with adding the Cabinet-decision-in-writing requirement and the easier Lok Sabha disapproval mechanism - all designed to make declaring and continuing a National Emergency significantly harder than it was in 1975.
This is why, since the 44th Amendment, a government could not repeat the 1975 Emergency's pattern of a broadly-worded "internal disturbance" justification decided by the Prime Minister alone - a full Cabinet decision in writing is now required, and just one-tenth of the Lok Sabha can force a special sitting to vote on ending it.