It was introduced by the 1st amendment act along with article 31B. Art 31B states that the act and regulations mentioned in the 9th schedule shall not be deemed to be void in spite of an adverse judicial pronouncement on the ground that it violates or abridges any of the Fundamental rights. Hence insertion of acts in 9th schedule gives an immunity to the act from scrutiny.
Moreover article 31B is retrospective in nature, when a statue is declared unconstitutional by the court is later included in the 9th schedule. It is to be consider as having been in the 9th schedule since inception. In other words the judicial decision is nullified. This protection is not available to the amendments.
In Coelho vs State of Tamil Nadu 2007 the constitution bench unanimously held that the provisions of the act put under the 9th schedule after 24th April 1973 can be challenged in judiciary if they violate the basic structure of the constitution.