Text 3

aruruksor muner yogam
karma karanam ucyate
yogarudhasya tasyaiva
samah karanam ucyate

Translation

For one who is a neophyte in the eightfold yoga system, work is said to be the means; and for one who is already elevated in yoga, cessation of all material activities is said to be the means. 

Commentary by Srila Prabhupada 

The process of linking oneself with the Supreme is called yoga. It may be compared to a ladder for attaining the topmost spiritual realization. This ladder begins from the lowest material condition of the living entity and rises up to perfect self-realization in pure spiritual life. According to various elevations, different parts of the ladder are known by different names. But all in all, the complete ladder is called yoga and may be divided into three parts, namely jnana-yoga, dhyana-yoga and bhakti-yoga. The beginning of the ladder is called the yogaruruksu stage, and the highest rung is called yogarudha. 

Concerning the eightfold yoga system, attempts in the beginning to enter into meditation through regulative principles of life and practice of different sitting postures (which are more or less bodily exercises) are considered fruitive material activities. All such activities lead to achieving perfect mental equilibrium to control the senses. When one is accomplished in the practice of meditation, he ceases all disturbing mental activities. 

A Krishna conscious person, however, is situated from the beginning on the platform of meditation because he always thinks of Krishna. And, being constantly engaged in the service of Krishna, he is considered to have ceased all material activities.

Commentary by Sri Visvanatha Cakravarti Thakur

"But then the practitioner of astanga yoga would have to practice niskama karma yoga for his whole life."

This verse therefore speaks of the limits of karma yoga. For the practitioner of yoga (muneh), desiring to rise to the stage of steady meditation (yogam), the cause of his elevation is action, karma, because that purifies his heart. For one who has attained steadiness in meditation (yoga arudhasya),   the cause of maintaining that level is cessation (samah) of all actions, which produce agitation. In other words,  the aspirant  for steady meditation does not have complete purity of heart.