My current understanding of this metric is that it compares the first half of an effort to the second half using the recorded power out and heart rate info. For long duration efforts, I would expect to see the decoupling number to increase, except for (maybe) those super-fit individuals.
I used a power meter head unit to record a recent KoS Quest from start to finish. My Pw/Hr decoupling for the 9:26 hour effort was 17.4%. I certainly will not argue that the second half of my effort was more exhausting than the first half, but I wonder where to go from here, specific to what is available on Systm. I understand that KoS Quest numbers are (probably?) unique due to a ten-hour effort as to a one-hour effort but looking back on many of my previous one hour/two-hour SYSTM rides/rides on other platforms, I am seeing positive Pw/HR decoupling numbers (above 5). Is this simply a matter of needing more endurance rides, more rest between efforts, or am I missing the big picture with Pw/Hr decoupling?
Hey Sir @Rick, I have no idea 
but just wanted to let you know I moved your post out of the From the Coaches sub-topic to the broader Training topic. From the Coaches is meant to be posts from them not questions to them.
I’m tagging Sirs @Coach.Mac.C @Coach.Spencer.R @Coach.Jeff.H and Dame @Coach.Suzie.S to see if they might have a comment or two on the question.
I’ll let the coaches provide the authoritative answer, but my understanding is that the higher the duration / intensity is relative to your fitness level, the higher Pw/Hr values you’ll see (same pattern for Pace/HR for running).
There are many ways to get fitter and they should all help reduce the Pw/Hr values. However, the old saying applies here: “It doesn’t get easier, you just get faster”
Maybe the Pw/Hr value is a good metric for mental toughness, showing you how deep you’re willing to suffer for … 
Answering from a different angle though … Pw:Hr is a good metric to gauge whether you were in an endurance training zone on a ride.
Theoretically, this number would be around 0% if you were truly below your 1st ventilatory threshold / 1st lactate turn point.
So there are a few things that contribute to a rising HR in relation to watts.
There’s heat, which increases the need for blood flow to remove the heat along with the fact that as you get warmer your body tends to use more carbs, I’ve seen some literature that as duration increases as well as heat and intensity, it can lower your LT as you ride, thus leading to greater cardiac drift. With an effort that length, and with the varying intensity there are a few things that could help. Training to improve aerobic fitness (here) as well as improving capacity to switch between using different fuel sources (fat and carbs). Often when you ride hard you switch to carb usage but it can be difficult for the body to go back to burning fat. Again why the HR can stay elevated compared to power
I’ve gotten interested in decoupling as a metric for endurance level fitness. What I’ve found over 3 years of using SUF is that the relatively short, high intensity workouts give me decent top end (MAP/VO2 max) coming into the spring racing season, but I lack the base miles that allow me to go longer at intensity (haven’t built up all those lovely mitochrondria spending a lot of time in Z2). So my strategy has been to come into the spring and do some criterium racing (which is about 30 minutes long for my category), then begin to add some longer, slower rides as the weather and time allow (I’m in Minnesota so Spring really just got here last week). Here’s a look at a ride I did Saturday which was the longest I’ve done so far this season. It was hilly, and it got warm. I know these things contribute to cardiac drift. What I was really looking at on this data was the decoupling ratio for the middle part of the ride, which actually looks pretty good to me. My hope is that over time the % of the ride I can do with a low decoupling ratio will expand, especially as I get more miles in at that lower Z2 level:
I’ve run into the same situation, my doing, via a DYI training scheme. I have been concentrating so much on my 4DP MAP weakness that I have not been getting enough base miles at lower intensity. I’ve been trashing my legs every day with no energy left to even consider longer rides during the week. Now the longer rides are more of a struggle - holding FTP in the middle/end of rides is more challenging than it should be. So I am currently trying to get more base miles in and strike a better balance on going after 4DP weak areas. On a side note, all that time in Ergo mode also meant that I lost my proficiency in knowing the proper gearing/cadence that I need to be in going on climbs IRL, which has led to too many over-spinning/mashing the pedal scenarios
Don’t do that! You get stronger when you rest after working out. If you’re crushing yourself everyday, you are probably not making all the gains that are possible.