This is bodybuilding for the heart and it's all about building capacity
In these sessions, you’ll often find a really low rep scheme and time under tension for each movement. This is so that we aren’t limited in the intensity we can put out because of muscular fatigue or muscular endurance.
In these sessions, we never want to take the muscle to total failure. We are testing and training the heart to see how well it can pump oxygen around the body, rather than how well the muscles can utilise it when it reaches the muscle.
The work-to-rest periods for such short rep ranges are perfect to make sure the intensity is maintained throughout the sets, giving the heart and lungs time to recover the rise up to their limits again.