It is the special power by which we do all Divine work, even forgiving. When you say, "Mother forgive us", the technique by which I forgive you is Nirmala Vidya. The technique by which I love you is also Nirmala Vidya. The technique by which all the mantras are manifesting themselves and are effective is also Nirmala Vidya.
Nirmala means pure, Vidya means knowledge. Nirmala Vidya is purest knowledge or the knowledge of this technique. It creates loops, the energy creates loops and creates also different formations by which it acts and draws all that is unwanted, impure, and fills it up with its power. ...
How subtle it is. Only by saying Nirmala Vidya, you just invite that power, the whole thing, the whole technique to attend to you and it attends to you. You dont have to worry. ... That technique is called Nirmala Vidya. That technique once mastered, by surrendering into it, it completely obeys, absolutely.
H.H.Shri Mataji Nirmala Devi, extract from Talk at Rahuri, Maharashtra, 31/12/80.
Full transcript in Nirmala Yoga no.1, pp20-21