In plain words
When dirt gets onto this same thing, they call it dirty cloth. In the same way, when we fill the heart with impure feelings, it turns into Manas.
The man who takes Manas as his support falls lower than a beast and comes to ruin; the man who has taken citta as his support becomes Paśupati: so said the Lord of Parthi.
Tiruttoṇḍa Āḷvār too used to say this very thing. "With a pure citta I am offering worship, O Rāma." So if we want purity of citta, we must develop śravaṇa, manana and sight so that they become pure. When we do this, the mind and the senses turn inward and stay pure, and we can experience the nāda-bindu-kalā.
Then the fifth, śāmbhavī mudrā. This takes the five organs of action, the five organs of knowledge and the four principles of the antaḥkaraṇa, namely Manas, Buddhi, citta and Ahaṅkāra. These fourteen we must turn inward. How is this to be done? This Manas, Buddhi, citta and Ahaṅkāra. Ahaṅkāra, citta, Buddhi, Manas: in this way we must establish it in our heart, step by step. When we root out Ahaṅkāra, purity of citta comes. When purity of citta comes, the flowering of Buddhi shows itself. Only when Buddhi shines does the mind shine. This pure mind alone brings both the organs of action and the organs of knowledge under its control. So first of all we must root out Ahaṅkāra. Ahaṅkāra too is simply the working of a thought. We may be proud because of some property. We may be proud because of wealth, because of strength. We may be proud of our beauty. We may be proud of our learning. We may be proud of our own intellect. But these forms of Ahaṅkāra are bound up with evil vāsanās. No man is to be found in this world who does not yield to these ten senses. So when we want to get away from these ten senses, we must gather to ourselves sacred feelings and the rooting out of Ahaṅkāra. Suppose all three are there: learning, saṃskāra and intellect.