It’s kind of a simple way of putting it but if you experience more cardiac drift at the end of a long workout it means there is a trigger for adaptation and you become stronger?
Can anyone prove this thought? I heard it from a coach but can’t verify.
Not so sure an adaptation “trigger” would present in such an obvious and immediate manner; the body doesn’t change acutely.
I would think it reveals aerobic weakness (or strength), more than anything. If you’re good at 3hrs but really drift on a 5hr ride, that would be a clue of where you require more training.
Look at it like a very long interval. For example, if you always blow up near the end of a 3min VO2 interval, that doesn’t mean you’re getting stronger.
Then again, I’m not a physiology whiz so I could very well be off the mark. 
@chrismilk was the coach trying to describe aerobic decoupling? If so, that’s just describing that if your drift exceeds 5% for the second half of an AeT ride (55-75% FTP/zone 2-ish) compared to the first half then you are not considered trained enough for that duration. Perhaps the coach was trying to say if that happens you need to ride more base to stay coupled for the duration you are targeting which would be by definition adaption. Once you are coupled then it’s on to build…