Back balance (BB) is a static posture and balance swim drill performed similarly as the front balance (FB) drill just on your back. You do not need to worry about breathing because your face is exposed to air the entire time. It is also easier to work on kicking from the hip with straight legs - not from the knees or engaging the quad muscles in you legs.
Push off the wall on your back with both arms at your sides, head aligned with your spine, and nose pointed up and breathing freely. Begin kicking easily, from the hip. Lightly press your chest buoy (lean on a spot between your shoulder blades) toward the bottom. Check for the tight line of good aquatic posture. This may feel different when you are on your back than when you were on your front.
You may notice a tendency to tilt your head back and to arch your back. Both these motions will work against balance. Using abdominal tension to flatten your lower back and extending your neck and bodyline may seem counterintuitive until you have practiced it a few times.
1. When you are in balance, the water will surround your face. Your ears will be underwater. The waterline around your head should be at the tip of your chin, at the crest of your forehead, and equally on both sides of your goggles.
2. Your pelvis should be at or within an inch of the water surface.
3. Your knees and toes should barely break the surface of the water as you kick from your hips.
After you have spent some time using the BB drill with good aquatic posture, experiment as you did with FB -- purposely relax into a schlumpy posture and note the effects. The draw yourself back into a tight line. You will likely experience the same problems with schlumpy poasture as you did in the FB drill, as well as the benefits of regaining good, tall posture.