This is an old Tibetan technique of humming, which
  creates a healing vibration throughout the body, and
  a hand movement which centers the energy at the
  navel.
  Nadabrahma meditation lasts for one hour and has
  three stages. It can be done at any time of day,
  alone or with others, but have an empty stomach
  and remain inactive for at least 15 minutes
  afterwards.
 

      First stage: 30 minutes

      Sit in a relaxed position with eyes closed and lips together humming loudly enough to
      be heard by others and create a vibration throughout your body. You can visualize a
      hollow tube or an empty vessel, filled only with the vibrations of the humming. A
      point will come when the humming continues by itself and you become the listener.
      There is no special breathing and you can alter the pitch or move your body
      smoothly and slowly if you feel like it.

      Second stage: 15 minutes

      The second stage is divided into two 7 1/2 minute sections. For the first half, move
      the hands, palms up, in an outward circular motion. Starting at the navel, both hands
      move forwards and then divide to make two large circles mirroring each other left
      and right. The movements should be so slow that at times there will appear to be no
      movement at all. Feel that you are giving energy outwards to the universe.

      After 71/2 minutes turn the hands, palms down, and start moving them in the
      opposite direction. Now the hands will come together towards the navel and divide
      outwards to the sides of the body. Feel that you are taking energy in. As in the first
      stage, don't inhibit any soft, slow movements of the rest of your body.

      Third stage: 15 minutes

      Sit or lie absolutely quiet and still
 

source: http://www.osho.org/meditate/applets/nada.htm.
 

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