Ive only just been learning and using this, so Ill give you my take on it but Im prepared to be educated on it as well.
SLowman is right when he says you find your maximum sustainable heart rate, and work off this. Above this heart rate is where you will start dragging in big breaths and lactic acid starts to build up in the legs. Up here your energy use exponentially increases and you burn the Glycolic energy stored in your body very quickly. Not a good thing for enduros apparently.
I used the CTS field test to figure out my Maximum sustainable heart rate (MSR).
Find a flat road (or use your wind trainer) and after a 20 minute warm up with little spikes in power, do an 8 minute time trial. I.e., go as fast as you can sustain for 8 minutes. Start your heart rate monitor timer and gun it, sustainably. Get your average heart rate over the 8 minutes.
Have a rest for 10 minutes, easy peddling, then do it again. This is the last one, so give it all you have.
In the end, you will have 2 average heart rates, and the highest of these is the maximum sustainable heart rate. (MSR)?
For example, mine is 158 bpm.
It might be called your Aerobic Threshold, or lactic threshold, either way, once you know it, you will consistantly notice that when your heart rate hits it, your lungs and legs start to blow up.
What difference does it make? Well, when training, you work off this number to acheive different goals.
Hill climbs, maintaining your heart rate at +90% of your MSR is good for increasing aerobic fitness. In training I try to climb hills with my heart rate between 150 and 158 which is right on my max. The benefit is, I know not to go over 158 or I will be burning too much fuel, but at the same time, maximising my fitness. Perhaps its the most efficient way to get up the hill energy wise?
Doing interval training, which helps increase your lactic threshold, VO2 max, and other measures of power, you need to do intervals above this MSR. SO, for example, Im on a training regime where I do 2 sets of 3 x 2 minute intervals (Power intervals) with a 2 minute rest between reps, 10 minutes between sets. I need to have my HR above 158 for those 3 minute intervals. Its harder to do than you might think.
Etc etc, different training sessions targeting different strengths.
But again, why bother? You know when your body has reached max, and you probably already work to this. Well, for me, knowing I have an upper limit and that to do a 24hr I need to conserve Power, its good to get used to training at just below the MSR amd be alereted to times when it inevitably goes above this level (climbing).
Also, the longer endurance or tempo rides, where you try to keep at say 85% of MSR, really help to increase aerobic fitness without blowing up. I ride above 140 but below 150 for these long sustained periods.
Enough of my rant, but I am finding it very useful.
Hope that helps
Raf