To improve cardio, jiu jitsu athletes will often do interval training of going hard for 20 seconds and light for 10 seconds at whatever exercise like running, kettlebells, or bodyweight stuff over and over.
In theory that simulates a jiu jitsu match. In reality, it simulates a jiu jitsu match if your technique, confidence, and strategy are all solid. Most matches at the lower ranks seem to be balls-to-the-wall non-stop with a gradual decline in intensity as both guys become progressively more exhausted. The best guys know when to relax, so that they recover properly for the bursts of intense effort needed to improve position, finish a submission, or just win a scramble.
And I’m not talking about stalling. In fact, stalling can often take up a lot of energy as well. I’m talking about relaxing whatever body parts that are not needed in the current exchange. That sounds kind of weird, but it’s the way I think about it. For example, I try to maintain a dynamic side control where I’m very heavy but completely relaxed. If my opponent decides to go crazy, I will expend energy as well to maintain the position, moving around to north-south, knee-on-belly, or even back to a bad guard that I can pass again right away. But I always try to spend significantly less energy than my opponent.
Of course, all that is easier said than done. Relaxing requires an understanding of a lot of details involved in maintaining and improving the position. If my opponent does something that makes me nervous, I’ll tense up and use energy to hold on just like I did on the first day of training as a white belt. The more I learn, the more confident I become in the fundamentals of good base, grips, posture, etc. With this confidence comes that ability to relax amid chaos.
It’s a weird balance to try to strike between the competition intensity of “win at all costs” and the need to relax at any good opportunity.