Bhashya
यो ह वै प्रतिष्ठां वेद, प्रतितिष्ठत्यनयेति प्रतिष्ठा, तांप्रतिष्ठां प्रतिष्ठागुणवतीं यो वेद, तस्य एतत्फलम् ; प्रतिति-ष्ठति समे देशे काले च ; तथा दुर्गे विषमे च दुर्गमने चदेशे दुर्भिक्षादौ वा काले विषमे ।यद्येवमुच्यताम्, कासौप्रतिष्ठा ; चक्षुर्वै प्रतिष्ठा ; कथं चक्षुषः प्रतिष्ठात्वमित्याह —चक्षुषा हि समे च दुर्गे च दृष्ट्वा प्रतितिष्ठति ।अतोऽनुरूपंफलम्, प्रतितिष्ठति समे, प्रतितिष्ठति दुर्गे, य एवं वेदेति ॥
In plain words
Whoever truly knows what a foothold is — knows that it's called that because "one stands firm by means of it" — whoever knows that quality as his own gets exactly this: he stands steady on level ground and in easy circumstances, and just as steady on uneven ground, on terrain that's hard to cross, in a place hit by famine, or in any hard season. But then — what actually is this foothold? The eye is the foothold. How can the eye be that? Because it's by seeing with the eye that a person keeps their footing, on smooth ground and on rough ground alike. So the reward matches exactly: he stands firm on level ground, stands firm on rough ground, simply by knowing this.
Close to the Sanskrit
He who indeed knows the foothold [knows that] 'one stands firm by means of this' — thus [it is called] the foothold; that foothold, possessed of the quality of being a foothold, he who knows — his is this fruit: he stands firm on level ground and in [smooth] circumstance; likewise on uneven ground and in terrain hard to traverse, and in a place [afflicted] by famine and the like, or in a difficult circumstance. If it be so said, [one may ask]: what is this foothold? The eye, verily, is the foothold. How is it that the eye has the nature of being a foothold? To this he says: because, having seen by means of the eye, one stands firm both on level ground and in difficult terrain. Hence the fruit that matches: he stands firm on level ground, stands firm in difficult terrain — he who knows thus.