The idea behind the "fat burning zone" is that if you do steady state cardio at a lower intensity that you will be burning fat more efficiently than you would be doing more intense cardio for the same amount of time.
This idea is based off of what is called the RQ, or respiratory quotient. The RQ is a measure of the amount of cardio dioxide expired during exercise divided by the amount of oxygen expired. When the RQ is utilized along with a few other measures it tells us how much of our energy is coming from carbs and how much is coming from fats.
If you look at the chart above you can see that an RQ of 1.00 results in 100% of the calories you're burning being burned from carbohydrates (%CHO) and 0% of the calories being burned from fats (%Fat). Conversely, an RQ of 0.70 results in 0% of your calories being burned from carbs and 100% being burned from fats.
It's important to note that an RQ of 0.70 only occurs while the body is at rest. So this means that as the RQ increases from 0.70 to 1.00 the intensity of exercise also increases. At an RQ of 1.00, during very high intensity exercise, 100% of the body's fuel is coming from carbs.
So, out of this logic is born the idea that in order to burn more fat calories it makes sense to perform steady state cardio at a lower intensity. Now, while it is true that at a lower intensity you will be burning more fat than carbs, it is also true that at a higher intensity you will be burning more fat overall.
Here is an example with numbers:
Person A walks on a treadmill at 3.0 mph for 20 minutes.
This results in an RQ of 0.80 which translates to 67% of energy coming from fats and 33% coming from carbs.
At this pace, Person A burns a total of 4.8 calories per minute which translates to 96 total calories.
Of these 96 calories 67% are burned from fat. This equals 64 calories burned from fat.
The remaining 32 calories are burned from carbs.
So, as you can see, Person A burned more fat than carbs.
Now, Person B runs on a treadmill at 6.0 mph for 20 minutes.
This results in an RQ of 0.86 which translates to 46% of energy coming from fats and 54% coming from carbs.
At this pace, Person B burns a total of 9.75 calories per minute which translates into 195 total calories.
Of these 195 calories 46% are burned from fat. This equals 90 calories burned from fat.
The remaining 104 calories are burned from carbs.
So, even though Person A burned more fat than carbs, Person B burned more fat overall and, as a result, had a more efficient workout in the same amount of time.
Image - "5 More fitness Myths That Won't Go Away" by ACE Fitness, LINK
Content - NASM Essentials of Personal Fitness Training. Michael A. Clark, Brian G. Sutton, Scott C. Lucett. 2014. Chapter 4, Exercise Metabolism and Bioenergetics. 79-81.