What makes a good chin?

I think part of it is psychological. Fighters that aren't scared to get hit and don't flinch and react properly are less glass jawed. The more you brace for the punch, the better. If you kind of flinch instead, it's worse. If you react intelligently and brace properly by tightening your neck and pushing your head down against your neck, you can withstand the punch better because your head will not swing as wildly knocking the brain around.

 

You can train chin but only to a small extent - It's down to neck muscles to prevent your head moving however even if the head doesn't move the brain can move around inside the skull.

There are 2 physiological reactions to getting punched in the head which "hurts" you.

1. "The button" - A spot on the chin with a big nerve cluster (mandibular branch), on many people if you tag this it will shut them down. How they respond to getting ragged on the button is different between people. Otto Wallin hurt Fury in round 12 by tagging him with big left hook on the button.

2. The jerk of the head which causes the brain to hit the inside of your cranium. This can be dulled if you work your neck muscles. This is why bigger men tend to be able to take a puncher better. It's harder to snap their head around.

The recovery from getting tagged in the button or having your head snapped around and your brain getting shutdown is totally genetic. Some people are just natural freaks - Fury and Ali for example.

After Fury got up in round 12 he actually got hit with a MONSTER left hook but he took it like a boss. The guy is genetically programmed and built to fight. A born and bred fighting man

 

That's pretty much it. Fighters with shaky mandibles either have adapt, like say, Lennox Lewis or Wlad Klitschko did, or find some way to work on strengthening their chin muscles, as some have argued. The former strategy is typically more successful and widely utilized among glass jawed fighters because, as you suggest, the old adage "you can't train a chin," is largely true.

 

They tried to deceive the public with Amir Khan back in 2012 before he fought Garcia. They were blaming the weight for his inability to take punches during the Garcia fight literally seconds before Garcia clocked and dropped him. There is no way around a poor chin other than tweaking fighting styles and gameplans. It didn't work for Amir Khan because he's a basic fighter. His speed makes him look confusing to his opponents simply because of their inability to react fast enough to catch him even though he's predictable and wide open. That's why it usually takes 3 rounds+ for him to slow down to finally be able to successfully time him when he predictably hops in with one twos.