Section 84 IPC and the Myth of Irresistible Impulse

Ab kya hua? I'm so confused with Section 84 IPC. Yeh doctrine of irresistible impulse kaisa hota hai? Apparently, it's a defence for a person who commits an act which is otherwise not a crime because he was unable to resist the impulse. But here's the catch - the person has to have a disease of the mind, and this impulse has to be one that no normal person could resist.

But then comes the twist - if the person knows the nature of the act and that it is wrong, then this defence doesn't apply. Okay, but what if the person knows it's wrong, but still can't resist the impulse? Isn't it a bit contradictory then? Like, if the person knows it's wrong, they must be sane, right? But if they're sane, then they should be able to resist the impulse... or so I thought.