The affix yat comes in the sense of 'a bond' after the word hṛdaya being in the Genitive -6th-Case construction, when the word denotes a 'hymn'.,
The word हृदयस्य is understood in the sutra. The word बन्धन gives the sense of the affix and is qualified by the word ऋषि ॥ That by which a thing is bound is called बन्धनं ॥ The word ऋषि means here the Veda or the Vedic bymn. Thus हृदयस्य बन्धनमृषि = हृ꣡द्यः 'the Mantra by which the heart of another can be brought under one's control', i.e. a वशीकरण मन्त्रः ॥,