About this passageNear the end of Yama's teaching, and one of the earliest definitions of yoga in the literature: not a posture or a discipline of the body, but the steady holding back of the senses. The second verse adds a caution that is easy to miss — that this state is not a possession. It arrives and it leaves again, and the watchfulness is part of the practice rather than a sign of having failed at it.
When the five organs of perception become still, together with the mind, and the intellect ceases to be active: that is called the highest state.
This firm holding back of the senses is what is known as Yoga. Then one should become watchful, for Yoga comes and goes.
In plainer words
When the five senses go quiet, and the mind with them, and the reasoning stops working away — that is the highest state. Holding the senses steady like that is what yoga means. And then stay alert, because it comes and it goes.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.