‘Knots of the heart’ stand for tangles of impulses of desire and egoism.
The word in the Mundaka is singular, ‘the knot of the heart’, but two other Upanishads, using the same metaphor, make it plural, ‘the knots of the heart’. The verse in Chandogya Upanishad, perhaps the oldest of the Upanishads, says:
When the food of the mind is pure, the essence becomes pure; when the essence is pure, memory becomes firm; when memory becomes firm, there is a falling away (prati-moksha) of all the knots of the heart.
The Katha Upanishad speaks of ‘knots of the heart’
In fact, Shankara commenting on all three passages explains the word as a plural meaning. (This does not mean that other commentators could not use the phrase ‘the knot of the heart’ to refer to the cosmic knot of Ignorance, which apparently entangles the Real with the Unreal.)
Returning to the Mundaka passage. The ‘knot’ stands for the tangle of impulses of desire and egoism. Some of them are fulfilled, and others are not fulfilled. In either case, they leave a subtle but dynamic trace on the causal body, which is so to say a seed-bed. If a desire is fulfilled, it is on some occasion, and then the trace left by it is called a sanskara; then it has a drive to realize itself again in that form. So it has a form. If a desire is not fulfilled, it remains a general impulse, and it seeks fulfilment in some form or other. Then it is called a vasana. For instance, the desire for taste. If food is eaten for nourishment, rationally and not governed by desire for taste enjoyment, then it does not form a sanskara or a vasana.
Food is not thought about except to fortify the body and mind. But if someone begins to think about sweet tastes for themselves, and devotes time and energy to get them, that would be a vasana. Then if he once tasted a specially attractive sweet, and began to think not so much of sweet things in general but how to get more of that particular sweet – that would be a sanskara.
This trivial example is given to illustrate the distinction (though the division is not always kept up). In fact there can be vasanas and sanskaras of torture and murder, as evidenced in the Roman so-called Games, in which sometimes 250.000 died every year in the arenas. In his commentary to Gita Ch. XVII, Shankara says that the whole attitude to life, technically called ‘faith, is made up of sanskara-s good and bad in that individual. This basis of life and thought can be changed, though it is usually a long process of unbroken application.
The distinction is not always rigidly observed. For instance, in Gita VI the yoga training in one birth is said by S’ankara to form samskaras- which have a dynamic force in a subsequent birth, ‘carrying him forward even against his will’; whereas in the text Atma-bodha. Knowledge of Self, probably also by S’ankara, the yogic training is said to form a vasana with a similar effect. Vasana does sometimes come to mean a complex of Sanskara-s
It may be noted that in the case of powers arising from samadhi made to acquire them, the exercise of them carries in it a sort of contradiction. To effect the samadhi, memory must have been purified of all associations ( sutra 1.43, and 1.44 ). But after a success, the excitement will rouse all the latent desires, and it will become increasingly difficult, and finally impossible, to free the memory from them.
So the power will be lost, except in one who exercises it without any interest in it of pride or advantage. Sankara in his Brahmasutra commentary says that all these powers are ultimately dependent on the Lord.