My health is no longer visible, my heart shows up broken, and the pay to revive window is there. However, I'm still running around, dealing damage and picking up treasure. I can even pick up health (which makes the revive window go away), though I can't use healing items. This continues until I get hit a few more times, and I've witnessed this a couple times in the clockworks.
I would expect that if I were dead enough to be revived, I would also be dead enough to be... dead.
It happens when your health is lower than the last half of a bar, but is not yet 0.