Why do this drill?
Finger trail is a recovery phase drill that is used to develop a relaxed and well executed recovery. If done correctly the swimmer will have to lead the recovery with the elbow and have their fingertips and wrist below the elbow throughout the drill.
How to do this drill
At the start of the recovery phase do not release the hand completely from the water but keep the fingertips or half a hand in the water. Drag the fingertips forward through the water to the front of the stroke and then allow the wrist and elbow to enter the water. Your elbow should be ahead of the trailing fingers (be leading) during the drill and you should feel the pressure of the water against your fingers as you bring your hand forward.