Alternating between strenuous exercise and rest, based on the principal of overload, during exercise the heart is beating very fast, during rest heart rate decreases giving time to recover, used to develop anaerobic endurance, high intensity 90% of max heart rate.
Example to improve performance in speed swim
3 sets (6×25) front crawl
30 sec recovery between each length
5 min rest after each set
Perform training programme 3 times a week