Thanks to Pete the Fireman (aka Pete the Triathlete) for finding this for me.
Understanding Lactate Threshold Heart Rate Training by Marty Gaal.
For those of you doing endurance training, this is a way to push up your lactate threshold. That's the point at which your body produces more lactate than it can clear away. The result is fatigue. If you can push that threshold up, you can go harder, longer.