The "high elbow" is a technique during which you keep your elbow and hand high in the water during the pullback portion of the underwater stroke, vs an extended arm. It is used in competitive freestyle swimming to improve the swimmer's speed through the water. To develop the high elbow form, regularly practice targeted exercises that focus on the arm movements. Note that working with a qualified swimming coach is irreplaceable in identifying your strengths and weaknesses and improving your stroke.
Catch the Water
The high elbow technique is widely known as the "high elbow catch," as it's necessary during the early part of the stroke, when the swimmer is reaching out to "catch" the water and pull back on it, as if climbing along a rope or pulling back against a solid substance. By keeping your elbow high, you use your hand as the primary force pulling back on the water instead of your elbow. By contrast, if you drop your elbow down as you reach forward, you can't put the same propulsive power into the reaching and pulling motion of your hand.
Whether you're sitting at a desk or lying belly-down on the pool deck, you can perform out-of-water exercises to become more comfortable with a high elbow position. In an exercise devised by swim instructor Glen Mills, the first motion of the exercise is to extend your arm straight forward along the ground or your desk, straightening it at the shoulder. Next, keeping the palm of your hand in a flat, resting position, rotate your arm from the shoulder until your elbow is facing up. Again keeping your palm flat on the surface, return to the original position. Continue alternating between the two positions while keeping your palm still. As the movement becomes more natural, incorporate it into your freestyle stroke.
To incorporate the high elbow into swimming, swim coach Mike Bottom has swimmers on the Ann Arbor, Michigan Club Wolverine team visualize that their elbows remain as close to the water's surface for as long as possible throughout the stroke. To perform Bottom's exercise, swim the "human paddle" instead of the traditional freestyle, keeping your head out of the water so you can monitor your arm movements more carefully. Alternate the movement with laps of the freestyle, focusing on coordinating the high elbow with propulsive movement from your hips.
Tweaks and Tips
Once you've nailed down the basics of the high elbow and familiarized yourself with the movement through a few exercises, focus on the details of the form. Mills recommends doing the stroke at a much slower rate than usual, focusing on keeping the elbow over the hand. For additional feedback, have a qualified coach watch you as you do the move; a casual observer is unlikely to notice the relatively small variation in form between a proper high elbow and a more generic freestyle stroke.