The reason you keep the elbows high on the underwater pull is to increase power.
I hear this often from both coaches and swimmers. When one looks at the underwater shots of the world’s fastest swimmers, sprint or distance, one finds the recurring position of high underwater elbow, also called Early Vertical Forearm (EVF). The elbows are not just high, they are unusually high…almost in a contorted position with extreme extension (negative angle) of the shoulder joint, particularly when coupled with the body rotation in the opposite direction. it begs the question, can one really be stronger in this almost contorted position? I believe the answer is no. To test this, one can go in the gym and using the Free Motion pulleys, that many gyms now have, pull as much weight down with your arm relatively straight forward, then try it with your arm at the side, shoulder extended and elbow up. You will not be able to pull as much weight in that position. With the shoulder fully extended (negative angle), it is simply not in a good mechanical position of strength.
So if this weird high elbow position is not about power, what is it about? Drag. By changing the position of the arm as it moves through the pull cycle, one can reduce the drag coefficient significantly … not eliminate it. To prove this, kick with fins all out for 25 yards extending one arm above the head and the other straight down toward the bottom of the pool. You will soon learn how significant the drag of your protruding arm becomes when it is at right angles to your long axis. In fact, you will have to work to keep the arm in the position and with any speed at all, it will shake in the water like a palm tree in a hurricane in the Keys. Now try the same drill, but instead of putting your arm straight down, let it protrude straight out to the side but bend the arm 90 degrees at the elbow, as if you were swimming with a high elbow. You will feel considerably less drag in this position. Same arm…different position…a lot less drag.
Now I realize that this is not quite the same as while swimming, when only the upper part of the arm is moving forward throughout nearly the entire underwater part of the pull cycle (In order to cause frontal resistive drag, the object must be moving forward). However, the upper arm is also the largest part of the arm and changing it’s orientation in the water also reduces the drag coefficient. Achieving an EVF is simply maintaining the upper arm in a position closest to the line of motion and thus creates the least frontal drag.
The good news is that most coaches are telling you the same thing, pull with your elbows high underwater. Now you know the real reason.