Confused about Section 84 of IPC?

Main problem is textbooks explain it like it's some super-clear concept, but in reality, it's like trying to grasp a fist full of sand - it just slips away. I've been studying for CUET PG Law and still can't get my head around it. Can someone please share a clear, real-world definition or example of a Section 84 plea? Like, what actually happens in court when someone tries to plead this?

I read Ritesh Sinha vs. State of UP (1988) AIR 1505 where the SC held that "unsoundness of mind" is not the same as insanity. But still, how do judges decide when someone's plea is genuine or just a trick to avoid punishment? Any help would be appreciated, folks!