I have Garmin's HRM-RUN chest strap. It has motion sensors in it, on your chest. From that, the watch can tell how much bounce you have in your step. It also measures your stride length per step. So, for running, it calculates a metric called vertical ratio, which is a cost/benefit analysis of your upward vs forward motion. Because you turn your chest when you run, it also measures L/R balance. I have an imbalance (55/45) on the bike, but a perfect 50/50 split when I run. I think that implies a fit issue, but it's been the case on two different bikes.

I also have an HRM-SWIM. It has memory in the chest strap; it sends the HR data to the watch as soon as you get out of the water, and all the swimming I do has HR data to go with it.

I'm excited. I've had my Fenix 3 for 2 years now, and I've loved it. Even more great stuff inside now! I don't want to wait!