This yogic technique called tummo has been practiced by the monks and yogis of Tibet and Nepal for centuries. The technique increased one’s internal body heat.
From Source 1
WHY TUMMO MEDITATION TECHNIQUE IS PRACTICED:
This Tummo meditation is based on the ancient tantric teachings and the teachings are based on the common belief that we as a human beings are born with free flowing energy and we waste it by worrying about different sorts of things in our daily life. So, instead of worrying Tummo meditation is practiced to concentrate and burn all those worrying thoughts and keep our mind and body healthy. So, it is basically channelize all those worrying energy into productive ones.
HOW TUMMO MEDITATION TECHNIQUE IS PRACTICED:
Tummo is basically a breathing technique. We need to sit in a mat with your legs crossed over one another and with your hands placed over the knees. Then close your eyes, try to concentrate by kicking out all the thoughts out of your minds which are emerging from various directions. When we breath, the noticeable movements happens in our bowel, i.e., it comes up and down. So, concentrate on the movement of the bowel during the inhaling and exhaling periods. Slowly then start coming up and concentrate on your nose and feel the breath while inhaling and exhaling. Once you are able to concentrate without any straying thoughts, imagine your body as a hollow balloon and visualize that a hot lamp is placed at the center of that hollow balloon discharging heat. Slowly, your body will start heating up.
NOTE: IT WILL TAKE TIME STILL YOU MASTER THIS TANTRIC TECHNIQUE AND START GENERATING HEAT.
From Source 2
nner Heat Meditation (tum-mo)
From How to Meditate: A Practical Guide by Kathleen McDonald (Wisdom Publications).
Sit comfortably in your meditation place and generate a strong positive motivation for doing this inner heat practice. Determine to keep your mind relaxed, concentrated and free of expectations for the entire session.
Start by visualizing the central channel (shushuma) as a transparent, hollow tube, a finger’s breadth in diameter. It runs straight down through the center of the body, just in front of the spinal column, from the crown of your head to the base of your spine.
Next, visualize the right and left lateral channels (nadis), slightly thinner than the central one. They start from the right and left nostrils respectively, travel upwards to the top of\ the head and then curve over to run downwards on either side of the central channel. They curve inwards and join the central channel at a point approximately four fingers breadth below the level of the navel.
Take as long as you like to construct this visualization. Once it is stable, imagine a redhot ember the size of a tiny seed inside the central channel at the level of the navel. To strengthen this visualization, imagine reaching into a fire, taking out a tiny glowing ember and placing it in your central channel. Once it is there, really feel its intense heat.
Now, in order to increase the heat, gently contract the muscles of the pelvic floor, concentrating on the internal rather than the external muscles, and in this way bring air energy up from the lowest chakra (energy wheel or center) to the ember.
Next, gently take a full breath through the nostrils. The air travels from the nostrils through the right and left channels to where they enter the central channel just below the level of the navel. The air joins with the heat there and with energy brought up from below.
As you stop inhaling, immediately swallow and push down gently with your diaphragm in order to firmly compress the energy brought down from above: now the air energy is completely locked in, compressed from above and below.
Now, hold your breath as long as it is comfortable to do so. Concentrate completely on the ember in the navel area, whose heat is increasing and spreading as a result of the compressed air energy.
When you are ready, relax your lightly tensed muscles and exhale gently and completely. Although the air leaves through the nostrils, visualize that it rises up through the central channel and dissolves there. The heat emanating from the burning ember at the navel continually increases and spreads and starts to burn away the blockages at each chakra and starts also to warm the concentration of silvery blissful energy found at the crown chakra.
However, the focal point of your concentration is always the heat of the burning ember in the navel area. Once your first exhalation is complete, again tighten the lower muscles, inhale a second time, swallow and push down with diaphragm, thus again compressing the air on the heat, then exhale, releasing the air up the central channel once again.
Repeat the entire cycle rhythmically seven times altogether, the intensity of the heat growing with each breath.
At the seventh exhalation, imagine that the now burning-hot ember explodes into flames. They shoot up the central channel, completely consuming and purifying the delude energy at each chakra. At the crown, the flames finally melt and release the silvery blissful energy, which pours down the purified central channel giving pleasure at each chakra it passes.
Finally, when it meets the blazing ember at the navel chakra, there is an explosion of bliss. This blissful heat flows our to every atom and cell of your body, completely filling you, making your mind very happy.
Concentrate on this pleasure without tension or expectation, without clinging to it or analyzing it. Just relax and enjoy it.
You will notice that, no matter how strong the pleasure is, your mind and body are calm and controlled, unlike our usual experiences of physical pleasure when the mind is excited and uncontrolled.
If your mind should wander from its concentration to other objects – the past or future, objects of attachment or aversion- focus your attention on the subject of the thought, the mind that perceives the distracting object, the thinker. Watch the subject until the distracting thought disappears, then concentrate again on the blissful feeling.
Analysis of feeling: Having reached a state of clarity, it is good to use it to discover the nature of your mind. After concentrating on your feeling, being absorbed in it for some time, analyze it by contemplating each of the following questions. Take as long as you like.
Is the feeling permanent or impermanent? How? Why?
Is the feeling blissful or suffering? How? Why?
Is the feeling related or unrelated to the nervous system and in the mind? How?Why?
Does the feeling exist inherently, from it own side, without depending on anything else, or not? How? Why?
Examine each point from every angle. Finish the session by summing up your conclusion, and then dedicate any positive energy and insight gained during the meditation to your speedy enlightenment for the sake of all living beings.