It's a nice little party trick too. When iphones could first detect heart rates, some of my friends were talking about it, so I said that I could slow my heart rate down. None of them believed me, so I got them to measure it. Once it was done I asked them if they wanted it to be faster or slower. They still didn't believe me, so I did both.

It's one of those silly little things that isn't really useful, but can shut your friends up for a bit :p