maitrī-ādiṣu balāni
[Through Saṁyama on] on the qualities beginning with friendship the corresponding strengths will manifest
This in depth Saṁyama is in the realms of visualisation.
It involves concentrating on the qualities of friendship which Patanjali has already set out (sutra 1.33 refers) in an earlier sutra: friendship (maitrí), compassion (karuna), joy (mudita) and equanimity (upekshá). Through the development of these 4 qualities by visualisation exercises your mind gets purified.
Damian Cadman-Jones is an authorised teacher of Prana Vashya Yoga™