Dude, he explains how it works in the video.
The fact that the lady died at 12:15 is pure coincidence. (And I should note, we have only his word for that, and for the authenticity of the "death certificate".
The significant part of the "trick" is: he put the time (12:15) on a piece of paper in an envelope before he met her, and he made her pick the time 12:15 when asked for a random time by having her often look at a photo that showed a clock that read 12:15. The fact that she was not consciously aware of the clock (as she was looking at the lady) only makes it work better. When she was asked for a random time, her brain said "Hey, I've been seeing 12:15 a lot lately, let's use that."
What part don't you get?