That happened to me with 315 when I was going to do a negative with a 275 bench... I wrap my thumbs , and the bar did not slip out of my hand...
But my spotted let go before my arms were extended and I was supporting the weight , just helping to unrack trying not to burn out too much...
Well happens so fast I didnt have time to even try to slow the weight down or lift the weight , but the bar came down on my chest and did exactly what it did with him , bounched and then bounced on my throat... and immediately when it bounced on my throat , I pushed up as hard as I could , and the spotter also grabbed it at the same exact time...
Thankfully , despite feeling like I got punched in the throat for 15 minutes , no lasting effects , just alot of adrenaline.