Is it the point where quite simply put, the muscles you're pushing just switch off? As I've only pushed to what I'd consider absolute failure once or twice, and as an example one of these occasions was dumbbell press - the point at which I failed, my left arm was going to absolute jelly, shaking like crazy etc etc, then after what seemed like a lifetime (probably a few seconds of barely moving up, but not going down), it felt as though my arm just switched off and went limp, causing me to bounce the 34kg DB off my chest lol
Is that absolute failure? I mean, as far as I'm concerned without proper mental training there was no way I was completing that rep.