Max effort (100%) followed by a recovery until the heart reaches its resting rate.
Develops concentric heart. Teaches the heart how to beat faster. Builds muscle on the inside of the left ventricle.
Crucial for any activity that requires a brief max effort. But also important for a day-to-day living so the heart knows how to deal with super high heart rates.
If this is all you ever do, over time the left ventricle can get too small. Which then elevates the risk of a cardiac event.
But for this to happen you’d have to ignore every other type of cardiovascular conditioning.