In addition to what 2fst4u said, de-coupling is also a term used to describe something different from what you are addressing. You appear to be talking about heart rate increases or decreases over time periods such as weeks. As 2fst4u said, if everything else is equal your heart rate ought to decrease over time for a given speed if you are getting in better shape and more efficient.
Where I have seen "de-coupling" used is in the power meter world where your heart rate v. power is tracked within a given workout. During the workout, there is a point at which your heart rate will increase to maintain the same power. Another way to look at it is riding at a constant heart rate and tracking whether power decreases over the course of the workout. De-coupling will always occur at some point during a ride for any athlete if they are riding hard enough and long enough.
The WKO+ power meter software tracks this stuff using some type of algorithms. The guys who developed it (Coggen and Allen?) talk about looking for a de-coupling between heart rate and and power of 5% or less for certain rides. Once you get to that point, where de-coupling is 5% or less, it is of significance in their mind in regard to where you are in base training and signals you are ready for the more high intensity stuff.
I love to use all the fancy gadgets and have a power meter on my road bike, but to be honest for most of us it is probably really difficult to decide when to move on to harder workouts based upon data like that. If you've got an experienced coach, that is one thing. But otherwise you are probably just as well off by following a tried and true training plan and tracking how you feel. When that 2 hour, zone 2/3 ride starts to feel not too taxing, I'll bet you are ready to move on to the intense stuff and get ready to race.