Vishayvati vaa pravrittirutpanna manasah sthiti nibandhini
Vishayvati: Relating to subtle experiences and subtle elements
Pravritti: Nature (trait)
Utpanna: Emerged from
Nibandhini: To tie in, to still
Patanjali lists other ways to purify the chitta.
By stilling one’s state in a mind which is the product of subtle experiences and siddhis of yoga (chitta gets purified).
As one stills oneself in the one who has the siddhi of the subtler dimensions and their related experiences there is a transfer of shakti, which results in purification of the chitta. Only he who has experienced can grant you the experience, hence the role of Guru.