Section 84 IPC: Sab Ka Liya Khata Hai But Koi Bhi Nahin Chalta

I was re-reading Dhondu @ Chavan's case (2008 SCC OnLine Bom 416) and I'm just not getting it. According to Section 84, if the accused is insane, then they can't be held liable for their crimes. It's a great concept, but in practice, it's almost never applied. Like, what even is insane? Is it just having a mental illness or is it something more? And what about the burden of proof? Does the prosecution have to prove insanity or does the defence have to prove they're sane? Dhondu's case says it's the defence's burden but then what about the cases where the judge just decides? Like, I get it, it's meant to be a safeguard, but it feels like it's just a loophole that's not really used.