The body is a chariot. Four things ride in it — the senses, the mind, the discerning faculty and the Self — and everything depends on which is put where. Place all four. If it is wrong the chariot bolts, and you are told what a life assembled that way feels like.
This is the order the song gives outright: the senses are high, the mind higher than the senses, the discerning faculty higher than the mind — and beyond the discerning faculty, the Self. Every wrong assembly is a rung put out of place.
The chariot is older than the song. The Kaṭha Upaniṣad (1.3.3–4) sets it out first: know the Self as the rider, the body as the chariot, the discerning faculty as the charioteer, and the mind as the reins; the senses, they say, are the horses. The song assumes you know this — and then puts a charioteer in it who is not the intellect at all.
That is the turn worth noticing. In the eighteenth book the machine has an occupant the Upaniṣad's diagram does not show: the Lord seated in the heart of all beings, yantrārūḍhāni māyayā — turning them as though mounted on a machine.
18.61 the Lord in the heart →