This method is also known as LSD (Long Slow Distance) and LISS (Low Intensity Steady State). It is what most people think about when they hear the word “cardio”.
Spending a lot of times at lower intensities builds a strong aerobic system. In martial arts, it has been known for a long time that “miles build champions”.
Both boxing and muay thai have a very long tradition of using this method. In many fight camps in Thailand, they have a rule that you are only allowed to fight if you participate in the morning runs.
To use this method, you just need to keep your heart rate between 130 and 150 beats per minute for 30 to 90 minutes (Neumann et al. 2000, p. 146). You can use any activity that puts your heart rate in the right range.
Increase the weekly volume over time. Two sessions of 30 minutes each will have the same training effect as one 60 minute session. Pick a session length that fits well into your schedule.
The cardiac output method works by increasing the size of the left ventricle of the heart, which in turn increases the amount of blood that gets pumped with each stroke. That’s where the method gets its name from.
When your heart can pump more blood with each stroke, it lowers your resting heart rate. There is a correlation between faster endurance athletes and their heart size (Neumann et al. 2000, pp. 52-54).
You can use this method during your regular BJJ training. During sparring, use an easy flow rolling pace and don’t take any breaks. You have to keep your ego in check and stick to a slow and steady pace. That might mean that you will get submitted by people who would not catch you otherwise.
The opposite is true for the drilling. Have everything planned out beforehand and only drill techniques that you already know. Keep a steady pace and alternate with your partner every 1-2 minutes to keep your heart rate in the correct range.
References
Neumann G, Pfützner A, Berbalk A. Successful Endurance Training. Oxford: Meyer & Meyer Sport (UK) Ltd., 2000.