The Broken Arrow Drill can be used to work on numerous aspects of your swim stroke, including timing, hand entry and alignment. It's a great drill to incorporate into your warm up to help loosen off tight shoulders and relax the upper back.
The drill starts by kicking on the side. Straight away lift your top arm vertically and pause for a moment before "breaking" at the elbow and spearing the hand into the water. It may help to think of flopping your forearm down as your elbow bends. Use the stroke to rotate onto your other side and repeat.
The spearing-in motion of the drill can be very helpful for swimmers who have a tendency to splash their entire forearm down on hand entry, as it keeps the elbow higher than the fingertips and wrists.
Swimmers who struggle to keep alignment can use the drill to help improve posture. As the arm is vertical, think about sinking your shoulder blades down, then as the arm spears into the water, continue to draw your shoulder blades back and down. This focus on drawing back your shoulders will help to eliminate any crossover in your stroke.
Swimmers who need to work on stroke timing should focus on performing this drill with an almost "catch-up" timing. In other words, the outstretched arm only moves into the pull phase once the raised arm is just about to spear into the water. It may be helpful in this case to perform half a length of the Broken Arrow drill then gradually evolve the drill into freestyle for the remainder of the length, whilst maintaining the same rhythm and timing.