The side glide nose down (SGND) swim drill teaches you to cut a narrower path through the water thus decreasing resistance. This means that you want to swim freestyle as much as possible on your side and avoid spending time on your belly. The foundation of an efficient freestyle stroke is a series of alternating right and left side-lying glides, connected by snappy rotations of the body from one side position to the other. This drill will get you balanced and comfortable on your sides.
SGND is best done with a training snorkel, which allows you to swim complete lengths of the pool without worrying about turning your head to breathe. Push off from the wall while turned to your side, with your lower arm extended generally toward the far end of the pool, but at a slight down-ward angle. Begin kicking easily. Press your other arm firmly against your side. Point your nose straight down. lean on your chest buoy (the side of your chest or armpit). Maintain a good aquatic posture. Do this drill on both sides of your body.
As with the other balance drills, establishing correct aquatic posture in this drill may require some experimentation, but the fundamental point of drawing yourself into as tall a posture as possible still applies.
1. When you are balanced on your side, you will be able to feel a strip of flesh exposed to the air all the way down your arm from your shoulder to your wrist. Putting a bit more pressure on your buoy by leaning in on your armpit will help expose more of your arm to the air. That strip of flesh is an indicator of the position of your hips. If your arm is firmly pressed to your side and your writst is dry, then your hips are right at the surface.
2. Your extended arm should feel weightless at all times.
3. Note that when your posture is correct and you are balanced, your head should be in almost the same position as in the front-balance drill -- nose pointed straight down and just the back of your head exposed to the air.
4. If water enters the snorkel as you do this drill, it usually means that you have buried your head (which means that you've pushed your face toward the bottom of the pool instead of keeping it in tight-line posture and simply leaning on your buoy) or that you have buried your whole front end (by putting too much pressure on your buoy).
After you have spent some time with the SGND drill with a tight-line, experiment a bit -- purposely relax into a schlumpy posture for a few yards, then draw yourself back into good posture. Note the effects, both negative and positive, of each of these postures. Also experiment with the amount of buoy pressure you need to use in order to feel fully supported by the water. You will likely find that when you relax the posture tension in your core, the sensation of support is elusive or non-existent.