Dealing with Section 91 of the Indian Evidence Act

Dosto, I had a nightmare during my CLAT PG prep. I was trying to grasp the exceptions to the rule that admissions are considered relevant facts but not evidence (Section 80 of the Evidence Act). Suddenly, I stumbled upon Section 91, and my mind went blank. I didn't understand when it's applicable and what happens when it's invoked. After hours of studying, I came up with this practice question:

Question: X and Y were seen near the scene of the crime. Y later gives a statement to the police that X had warned him not to be there at that time. Is X entitled to use Y's statement against him under Section 91 of the Evidence Act?

I spent a long time preparing for this question and still managed to score 6/10. My weak areas are still understanding when Section 91 can be invoked and how it affects the credibility of the witness.