You can imagine floatation like this: Due to the completely relaxed floating situation in absolute silence and darkness (optionally with meditative music and/or very reduced light) all body functions drive down to a minimal level, the entire body is like switched off, except the brain which continues to operate on a different frequency.
Since the water temperature is equal to that one of the skin, we may loose the body surface sensation. The whole body can 'disappear' or it merges with the brine. Gravity perception disappears just because we don't feel any more pressure points on our skin. Under normal circumstances small areas of skin carry the body weight. The pressure perception suggests us our weight, means the gravity. During the floatation the entire back side of the body carries the weight, the pressure information disappears and can no longer be localized.
As we eliminate all stimulus information in this way, we almost automatically slide into a very deep state of relaxation. This can also be achieved with meditation, it takes just far more exercise.
About 90% of the brain capacity are used to process the information on these four stimulus levels (visual, auditive, temperature and gravity stimuli). So 90% of the capacity of the brain will not be used any more in the tank - or does it? Unlike the processor of a computer our nerve center will continue working on 'average load', so now it has capacity to process lots of our previous open issues (like dreaming!).
Through the illusion of weightlessness our musculoskeletal apparatus is freed from the burden of the body, the muscles relax at the maximum. The joints, intervertebral discs, ligaments and tendons are completely uncharged. Salutary effects can be set in record time.
The equilibria sympathetic <-> parasympathetic, endorphins <-> stress hormones, Yin & Yang adjust at new, well-being is achieved. After floating, you feel calm, refreshed and invigorated. You will be ready to continue your regular life with renewed energy and vigor.