Originally Posted by ericm979

It measures (or tries to measure) cardiac drift. Your HR goes up over time at the same power. It's worse, to the point where I noticed it just from watching the instantaneous numbers on the PM, when I am tired or do a ride that's too long for my current fitness... my power goes down but my HR stays the same.

The theory is that the more aerobically fit you are the lower the drift. But I am not sure how applicable it is, especially to an entire ride which has hard parts and not so hard parts. It sounds (from posts on Wattage list) like the research is pretty new and there isn't that much out yet.