No probs Kryton, it's not my tip, mind...
Now I only have a layman's understanding of this, and it may be out of date, I dunno, but this is what I think I know. The amount of fat you can 'burn' is limited by the amount of oxygen you can get to your muscles, so if you go over that power level your body will start to use glycogen stores. This needs less oxygen but generates lactic acid. Your blood and kidneys can remove this at a certain rate.
So once you have gone over this threshold (I think some people call it lactate threshold, lower lactate threshold or a variety of other things) you'll have at least some lactate acid in your muscles as it's being removed by your blood. Some lactate acid is tolerable but its presence causes fatigue in the long run. As you work harder you'll produce more lactate and eventually you'll produce more than your blood can remove and it'll start to accumulate. Above this level is sprinting, and that's why you can't do it for very long. I think this level is what people refer to as 'threshold' and above it is 'in the red'. The threshold (not in the red) is where you want to be (and will probably find yourself) on a long steady climb or a 10 mile TT. You can estimate where this is for you by doing a flat 10 miler or long climb and averaging your power, or averaging your heart rate for the last 20 mins of a 30 min STEADY effort.
I'm unclear on the chemistry so I am probably using terms like 'lactate' incorrectly.