Okay, let's try to simplify this. For those who really want to know the "why" I highly recommend the "Cardio Code" book.
For those somewhat interested in the why, this review of the Cardio Code by Andrew Reed highlights the main points:
"The Cardio Code" (Book Review)
For those who really could care less but are now "confused" by this thread, this paragraph from Andrew Reed's book review summarized the crux of the matter:
Following on from this topic, Jay looks at how much oxygen is used during various activities. I always thought if my heart pumped a certain number of times it meant I was working equally hard, regardless of the activity. But that’s not so, particularly when it comes to weight training.
During weight training, regardless of the lifts used, you simply can’t engage your muscles quickly enough to cause the action that takes place during
traditional cyclic forms of cardio. When muscles are engaged for long periods they actually cut off blood flow, which lowers the amount of oxygen able to pass through them and decreases the intensity of work you’re doing when compared to your VO2 max.
In other words, just having a high heart rate is not indicative of getting a cardio workout.
As Jay states in his book, “Having a high heart rate doesn’t mean you’re doing cardio.
If that was the case I could scare you into better shape."
For the really attention-span challenged, here is the key sentence: "When muscles are engaged for long periods they actually cut off blood flow, which lowers the amount of oxygen able to pass through them and decreases the intensity of work you’re doing when compared to your VO2 max."
So what? How is this practical? It's actually very practical. First, let's get our definitions straight.
"Cardio" means performing an exercise that improves heart and lung function. In order to improve heart and lung function you need to work at a certain percentage of your VO2max. Therefore, in order to do "cardio," you need to work at a certain percentage of your VO2max.
As
@Kettlebelephant noted, swings play by different rules. Yes they do. In the Cardio Code KJ had a diagram of a continuum. On one end, let's call it the left tail, you had "pure" anaerobic activity. Think a powerlifter performing a 1RM. On the other end, the right tail, you had "pure" aerobic activity. Think marathon running. Kettlebell swings fell somewhere in the middle.
Here's the practical part: you can move kettlebell swings along the continuum simply by changing weight. If you're doing heavy swings, you're closer to the left (powerlifter) tail. Your muscles are tense because of the heavy weight which means they are restricting blood flow. As a result of the blood flow restriction, the muscles are being powered by the anaerobic energy system. Use a lighter weight and you're closer to the marathoner. Your muscles may still have some tension, but not as much so some blood flows through. This allows some oxygen exchange which means that at least some of the muscles' energy is supplied by the aerobic energy system.