I'm not necessarily talking about riding at threshold... I often do isopower rides at tempo, or just below, and notice that my HR starts out fairly high (for the power), but drops steadily as I get warmed up. It may continue to drop slowly for the same power for an hour or two, then stabilize. Eventually, as I get tired, the HR will start to creep back up. That is reflected in the decoupling figures. Decoupling just means that the power and HR "lines" diverge. A negative number means that either HR is dropping for the same power, or power is increasing for the same HR, or some combination of the two. A positive numbers means the opposite.