Underwater work (specifically dolphin kick) is now considered the “5th stroke” for year-round swimmers, but for triathletes it still falls into the realm of a drill. Why would you want to spend time underwater when you’ll be swimming at the surface the entire time during your race? Being underwater improves your underwater competency. You’ll get a better feeling for how buoyant you actually are and how your breath affects your buoyancy. If you do dolphin dives at the beginning of a race you’re spending time underwater. Kicking can also be more effective underwater because your foot can manipulate more water since it’s immersed in water the entire time. Lastly, being completely submerged is faster than being partially submerged so you get to move faster underwater as long as you have a source of propulsion.
The main set today has all the sprints in the middle of swims. By sticking them in the middle it encourages you to be very efficient with your swimming BEFORE the sprint part, and after the sprint part you just have to survive to keep moving. You get to experience how hard it is to swim in an extreme state of fatigue. You also will practice changing speeds during a continuous swim which can help you differentiate your EZ speed from your sprint speed. The average triathlete has two speeds typically – moderate + and moderate ++. It takes a triathlete a while to learn what true sprint intensity is. I hope this workout helps.