Yoga Sutras 1.35
Patanjali, explained by Swami Vivekananda
विषयवती वा प्रवृत्तिरुत्पन्ना मनसः स्थितिनिबन्धिनी ॥
viṣayavatī vā pravṛttirutpannā manasaḥ sthitinibandhinī
Those forms of concentration that bring extraordinary sense-perceptions cause perseverance of the mind.
In plain words: Concentration that brings unusual sense experiences helps the mind keep going. The teacher says that strange smells, sounds, or sights from focusing end a beginner's doubts.
Swami Vivekananda explains
This naturally comes with Dhāranā, concentration; the Yogis say, if the mind becomes concentrated on the tip of the nose, one begins to smell, after a few days, wonderful perfumes. If it becomes concentrated at the root of the tongue, one begins to hear sounds; if on the tip of the tongue, one begins to taste wonderful flavours; if on the middle of the tongue, one feels as if one were coming in contact with something. If one concentrates one's mind on the palate, one begins to see peculiar things. If a man whose mind is disturbed wants to take up some of these practices of Yoga, yet doubts the truth of them, he will have his doubts set at rest when, after a little practice, these things come to him, and he will persevere.