people are trying so hard to figure out how this would work, and it's all failure so far.
In a horror game....what possible advantage would the game have knowing your stress level? Throw more enemies or screen shocks at you? It just doesn't fit at all.
So far, this is only useful for relaxation games. Stop trying to reassure yourselves that Nintendo is doing this to improve gaming, and start understanding that Nintendo did this to expand further into the casual market.
I'm sure something will come out and prove me wrong eventually, but really, your heartbeat isn't a fun control mechanic nor is it a practical one. It's not meant for "gaming".