There is an external physical side to three yoga procedures: sitting-in-meditation, controlling the vital forces by breathing, and repeating a mantram. But the physical is to make the mental process easier, not to distract from it. We know this in daily life, when we find we are tired and suddenly determine to go to bed.
Getting up, going to the sleeping place, changing into sleeping clothes, are all assertions of wakefulness. We may feel too tired to do them. But they are preparations for a deep and restorative sleep.
The preparations for meditation, though extravertive and active, are to make it easier to settle into an alert and aware introversion.
The traditional method of controlling breathing harmonizes the vital forces; the mantra harmonizes the feelings.
The psychological side of yoga is first of all to reduce unreal and unwanted feelings, mental twitchings, and inertia. When they are substantially lessened, there are glimpses of a Self, unlimited by any of them. Everything else is dying; this is the undying in the dying.
The second part of yoga is to stabilize the first flashes of vision, by giving up any remaining habits of clutching at things.