By always keeping the mind fixed on the Self, the yogi whose mind is subdued attains peace of the Supreme nirvana by uniting with Me.
A Self-realized person who is free from lust and anger, and who has subdued the mind and senses easily attains nirvana.
The person whose mind is always free from attachment, who has subdued the mind and senses, and who is free from desires, attains the supreme perfection of freedom from Karma through renunciation.
As a lamp in a spot sheltered from the wind does not flicker, this simile is used for the subdued mind of a yogi practicing meditation on Brahman.
In My opinion, yoga is difficult for the one whose mind is not subdued. However, yoga is attainable by the person of subdued mind by striving through proper means.