Udanajayat jala panka kantakadisu asangah utkrantih ca
“By mastery of udana vayu, the yogi can walk over water, swamps and thorns without touching them. He can also levitate.”
Vayu is usually defined as wind or air. Actually vayu is not merely air, but the medium in which air exists and the force by which it is held together.
The body functions are performed by five types of prana vayu, or vital energy, namely: prana, apana, samana, udana and vyana. Prana moves in the chest region (areas above the diaphragm) and controls breathing. Apana moves in the pelvic region and legs, it controls elimination of urine, semen and faeces. Samana moves in the abdominal region (between the diaphragm and navel), it controls digestion and maintains the harmonious functioning of the digestive and reproductive organs. Udana moves in the head and neck region, it controls the vocal chords and governs the senses, and respiratory system by forcing air out of the lungs. Vyana vayu pervades the whole body, governing relaxation, distribution of energy from breath and food, contraction of muscles, movement of joints, etc. Vyana regulates all processes of life.
Udana is the upward moving prana, it moves the energy from lower spine to the brain. By samyama on udana, lightness is achieved. Yogis who master udana can travel over water, mud or thorns without touching its surface. He can also make udana draws out the subtle body from the gross body, i.e. die at will.
Since udana goes from the throat upwards, it can be particularly stimulated by Jalandhara Bandha, or chin lock. Asanas which are good for udana include Setu Bandhasana, Viparita Karani, Sarvangasana, Matsyasana, and etc. Generally, inverted postures will be beneficial. In addition, chakras associated with udana include Vishuddha, Ajna and Sahasrara, practitioner may focus on these three chakras during meditation practice.
To become light (laghima) is one of the siddhis that a yogi may experience. This attainment does indicate that sadhana is on the right path; however, it can be an obstacle at the same time. As Patanjali highlighted in verse 3.38, practitioners should not be attached to the power gained, as this will finally crash them down. Always keep vairagya in mind, get over the attainment and obstacles, move forward towards kaivalya.