You can win a race before you take your first stroke. It’s that simple. The first 15 meters of your race is so important, because even a slight advantage gives you that mental assurance and power to finish. Also it is easy and free speed, with very little physical fatigue.
It’s 10% reaction speed, but 90% underwater kicking. It’s amazing how much faster we are underwater. https://www.youtube.com/watch?v=4EFjQDgIfKM.
For me, as a short swimmer (all my teammates tower over me), I need as many ‘shortcuts’ to offset the fact that all tall swimmers need to do is just pull to catch up to me in the water. By having a great start and going underwater 15 meters, it propells me forward at least 1 second, so that I have that advantage at the beginning of the race that all the other swimmers need to overcome before they can beat me.
My coach doesn’t really stress underwater or starts, however, I believe that this will be a easier way to improve your race times than just working that upper body like everyone else.