The specific practice called "Praises to the 21 Tara.
One might asks, "Why 21 Tara in specific? And not 22 or 23 or 40 or any random numbers?"
The answer to this question is beautifully clarified by Khenchen Palden Sherab and Khenpo Tsewang Dongyol in their book, "Tara's enlightened activity".
At the basic level, Buddha have taught us 21 techniques with which we are able to attain Enlightenment. According to the Mahayana Sutra system, as we practice we traverse the ten different levels (Bhumis), eventually reaching the enlightened state. The basis for our enlightenment is right where we find ourselves now, with the precious endowment of our own human body and our own buddha-nature.
Vajrayana or Tantra is similar to the sutra system, but its methods are more specifically targeted. According to tantric teaching, within this human body we have twenty one knots. These are in pairs and they obstruct or block our channels. Through practice, as we release each of these pairs of knots, we obtain a specific experience or realization.
After we have released all of the 21 knots, we are known as enlightened beings, having attained Buddhahood.
Buddhahood is not some force that is outside us, waiting for the knots to be untied in order to come in. From basic Buddhism all the way to Dzogchen, it is made perfectly clear that Buddhahood is an innate state, already within us.
Our inherently awakened state is an already enlightened being, a Buddha, Tathagatagarbha. When we release those 21 knots, we attain the ultimate awakening known as the Dharmakaya state.
The Dharmakaya, in turn has 21 spontaneously inherent qualities. They transcend duality, the compound state, permanence and impermanence, and effort or striving. Unceasingly, they arise as necessary for the benefit of all sentient beings.
These twenty one active Dharma kaya qualities appear as the twenty one emanation of Tara. Thus Tara combines all the active energies of the three kayas by which we release our own knots and those of other beings, the energy by which we achieve enlightenment and help other beings to achieve it.