I have heard lot about this mudra and its benefits and online text have different different version of it.
If any one can explain me in full detail with pictures, quoting the relevant texts and scriptures and how one can master it.
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Vajroli mudra is quite an intricate Yogic practice,It involves sexual intercourse too.
So,I don't really understand why you want to know about it from online resources.Or are you looking for some other Vajroli that i don't know about?
In anycase, in Hatha Yoga Pradipika,we find the following description of Dasa mudras including the Vajroli mudra:
Karani viparitakhya vajroli saktichalanam Idam hi mudradasakam jaramarananasanam
Viparîta Karanî, Vajroli, and Sakti Châlana. These are the ten mudrâs which annihilate old age and death.[Note-These mUdras have sub-divisions hence the 10 ]
Now the description of Vajroli Mudra:
Svechchaya vartamanoapi yoghoktairniyamairvina Vajrolim yo vijanati sa yogi siddhibhajanam
Even if one who lives a wayward life, without observing any rules of Yoga, but performs Vajrolî, deserves success and is a Yogî.
Two things are necessary for this, and these are difficult to get for the ordinary people—(1) milk and (2) a woman behaving, as desired.
By practising to draw in the bindu, discharged during cohabitation, whether one be a man or a woman, one obtains success in the practice of Vajrolî.
And,here are the benefits of practicing & mastering the Vajroli mudra:
Dehasiddhim cha labhate vajrolyabhyāsayogatah Ayam punyakaro yogho bhoghe bhukteapi muktidah
By the practice of Yoga consisting of the practices of Vajroli, bodily perfection is obtained (beauty, grace and great strength). This Yoga confers merit (punya), and though there is sensual experience, it leads to emancipation.
Also note that i have intentionally omitted the actual description because its of no use.One can't learn yoga by studying millions of shahtras.It is to be learnt only from a Guru.(This is what the Shastras declare & not something that i'm saying).
BTW-There is also a mudra in Yoga called "Vajroni".Are you by any chance seeking information on that one ?If yes then i'll update my answer later.