धीवशात्सत्त्वसिद्धिः
In plain words
From mastery of comes the of .
The commentators
Śivasūtra with Kṣemarāja's Vimarśinī
अस्य च - धीवशात्सत्त्वसिद्धिः ॥ १२ ॥ धीः तात्त्विकस्वरूपविमर्शनविशारदा धिषणा तद्वशात् सत्त्वस्य स्फुरत्तात्मनः सूक्ष्मस्य आन्तरपरिस्पन्दस्य सिद्धिरभिव्यक्तिर्भवति । नाट्ये च सात्त्विकाभिनयसिद्धिर्बुद्धिकौशलादेव लभ्यते ॥ १२ ॥
In plain words
Through the power of the understanding, the attainment of sattva. And for him: Dhī is the understanding skilled in reflective awareness of the real nature. Through its power there is the attainment, the manifestation, of sattva: of the subtle inner vibration, whose nature is pulsing radiance. And in a drama too, success in the sāttvika acting is gained only through skill of understanding.