Vajra is 'thunderbolt' or 'lightening'. It is also the weapon of Lord Indra means 'mighty one'. Vajra in this context refers to the vajra nadi which governs the urogenital system. It is the second innermost layer of sushmna nadi. Vajra nadi is the energy flow within the spine which governs the sexual systems of the body. In mundane life it is responsible for the sexual behaviour and this aspect has been termed 'libido' by Dr. Freud and as 'organe' by Dr. Reich. In tantric sadhana this energy is not suppressed but it is awakened and redirected. Oli mudras (vajroli, sahajoli, amaroli) are those which specifically sublimate sexual energy into ojas (vitality) and kundalini shakti.
According to Hatha Yoga Pradipika, if vajroli is well practised, even in an otherwise free lifestyle, that yogi's attainments in life will be greater, and a greater source of vital and mental power will become available to him. A few great yogis and masters had these experiences and have, therefore, instructed their disciples in the oli mudras and other Hatha yoga techniques.
Vajroli mudra is an important practice today in the kali yuga when man's ability and need to express himself in the material and sensual world is predominating. We have to act in the external world and simultaneously develop inner awareness. The purpose of life should be to attain a deeper and more fulfilling experience beyond the empirical sensory experience alone.
By the practice of vajroli one can learn to control the physical mechanism, but side by side with his mental control must also be developed. When the mind wanders in useless fantasy, energy is dissipated. When the mind is totally concentrated on a specific object, symbol or point, pranic movements are channelized and the bindu is maintained. In Hatha yoga the mind has to be kept on the aim of the practices in order to induce total one-pointedness.