In plain words
When one is no longer attached either to sense objects or to actions, and has become a sannyāsin of every saṅkalpa, one is said to be established in yoga.
Word for word
When indeed [one] clings neither to sense objects nor to actions, renouncing every saṅkalpa, [he] is then called ascended to yoga.
यदा हि नेन्द्रियार्थेषु न कर्मस्वनुषज्जतेसर्वसङ्कल्पसन्न्यासी योगारूढस्तदोच्यते
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
अथेदानीं कदा योगारूढो भवति इत्युच्यते —
यदा समाधीयमानचित्तो योगी हि इन्द्रियार्थेषु इन्द्रियाणामर्थाः शब्दादयः तेषु इन्द्रियार्थेषु कर्मसु च नित्यनैमित्तिककाम्यप्रतिषिद्धेषु प्रयोजनाभावबुद्ध्या न अनुषज्जते अनुषङ्गं कर्तव्यताबुद्धिं न करोतीत्यर्थः । सर्वसङ्कल्पसंन्यासी सर्वान् सङ्कल्पान् इहामुत्रार्थकामहेतून् संन्यसितुं शीलम् अस्य इति सर्वसङ्कल्पसंन्यासी, योगारूढः प्राप्तयोग इत्येतत् , तदा तस्मिन् काले उच्यते । ‘सर्वसङ्कल्पसंन्यासी’ इति वचनात् सर्वांश्च कामान् सर्वाणि च कर्माणि संन्यस्येदित्यर्थः । सङ्कल्पमूला हि सर्वे कामाः — ‘सङ्कल्पमूलः कामो वै यज्ञाः सङ्कल्पसम्भवाः । ’(मनु. २ । ३) ‘काम जानामि ते मूलं सङ्कल्पात्किल जायसे । न त्वां सङ्कल्पयिष्यामि तेन मे न भविष्यसि’(मो. ध. १७७ । २५) इत्यादिस्मृतेः । सर्वकामपरित्यागे च सर्वकर्मसंन्यासः सिद्धो भवति, ‘स यथाकामो भवति तत्क्रतुर्भवति यत्क्रतुर्भवति तत्कर्म कुरुते’(बृ. उ. ४ । ४ । ५) इत्यादिश्रुतिभ्यः ; ‘यद्यद्धि कुरुते जन्तुः तत्तत् कामस्य चेष्टितम्’(मनु. २ । ४) इत्यादिस्मृतिभ्यश्च ; न्यायाच्च — न हि सर्वसङ्कल्पसंन्यासे कश्चित् स्पन्दितुमपि शक्तः । तस्मात् ‘सर्वसङ्कल्पसंन्यासी’ इति वचनात् सर्वान् कामान् सर्वाणि कर्माणि च त्याजयति भगवान् ॥ ४ ॥
In plain words
Now the next question: when does a person actually become established in yoga? This is what is said in answer —
'When' the yogin's mind is settling into composure, he no longer clings — no longer feels any obligation — toward sense-objects, sound and the rest, or toward actions, whether obligatory, occasional, optional, or forbidden, because he sees no purpose left in any of them. That's the meaning. 'One who renounces all resolves': whoever's nature it is to let go of every resolve that drives him toward wealth and pleasure, here or hereafter — that person is 'one who renounces all resolves,' someone who has ascended in yoga, who has attained it; that's what he's called at that point. This phrase implies he should renounce every desire and every action, since every desire is rooted in resolve — as Manu says, 'desire is rooted in resolve; even sacrifices spring from resolve.' And the Mokṣadharma has this striking line: 'O desire, I know your root — you are born from resolve. I will not form a resolve for you, and by that, you will simply cease to exist for me.' Once all desire is given up, the renunciation of all action follows naturally, as the Bṛhadāraṇyaka says: 'as a man's desire is, so is his resolve; and as his resolve is, so is the action he performs.' And Manu again: 'whatever a creature does is nothing but desire acting itself out.' It also stands to reason — once every resolve is renounced, no one can so much as stir. So it's by that phrase, 'one who renounces all resolves,' that the Lord has him give up every desire and every action.
Close to the Sanskrit
Now, at this point, when does one become established in yoga? — thus it is said:
'When,' the yogin whose mind is being composed, indeed, does not cling, [i.e.] does not form the notion of obligation, with the sense that there is no purpose [left], in sense-objects, [i.e.] the objects of the senses, sound and the rest, and in actions, obligatory, occasional, optional, and prohibited — that is the meaning. 'One who renounces all resolves': he whose habit is to renounce all resolves, which are the causes of wealth and pleasure here and hereafter — he is 'one who renounces all resolves,' [i.e.] one who has ascended in yoga, one who has attained yoga; this, at that time, is he called. From the statement 'one who renounces all resolves,' one should renounce all desires and all actions. That is the meaning. For all desires are rooted in resolve — 'desire is rooted in resolve indeed; sacrifices arise from resolve' (Manu 2.3); 'O desire, I know your root: you are born from resolve surely. I will not form a resolve for you; by that, you will no longer exist for me' (Mokṣadharma 177.25) — because of this Smṛti text and others like it. And upon the abandonment of all desire, the renunciation of all action becomes established — 'as his desire is, so is his resolve; and as his resolve is, that action he performs' (Bṛhadāraṇyaka Up. 4.4.5) — because of this scriptural text and others like it; 'for whatever a creature does, that is desire's own doing' (Manu 2.4) — because of this Smṛti text and others like it; and by reasoning too — for once all resolve is renounced, no one is capable even of stirring. Therefore, by [saying] 'one who renounces all resolves,' the Lord makes him abandon all desires and all actions.