Underwater Recovery (aka Dog-paddle)
A key element to efficient swimming is learning how to slip through the water. To practice finding the smoothest path, swim the freestyle stroke but keep your hands and arms underwater during the recovery. Don’t lift your hand out of the water after it passes your hip. Instead, slide it up the side of your body, keeping your arms and elbows tucked in close. Move your hand forward into the reach position and then start another stroke.
Example set: 12×25 with 10 sec rest as 3x(2 Underwater Recovery/2 swim)