Link to purport by A C Bhaktivedanta Swami Srila Prabhupada

Transcript of Bhakti-Shastri class on this verse by Chaitanya Charan

Bg 18.57

cetasā sarva-karmāṇi
mayi sannyasya mat-paraḥ
buddhi-yogam upāśritya
mac-cittaḥ satataṁ bhava

Synonyms: 

cetasā — by intelligence; sarva–karmāṇi — all kinds of activities; mayi — unto Me; sannyasya — giving up; mat–paraḥ — under My protection; buddhi–yogam — devotional activities; upāśritya — taking shelter of; mat–cittaḥ — in consciousness of Me; satatam — twenty-four hours a day; bhava — just become.

Translation: 

In all activities just depend upon Me and work always under My protection. In such devotional service, be fully conscious of Me.

cetasā sarva-karmāṇi, Whatever Karma one might be doing, with our consciousness offered to Krishna 

mayi sannyasya mat-paraḥ, make Him the supreme goal of life

buddhi-yogam upāśritya, Krishna has used the word Buddhi Yoga several times, but each usages the meaning is getting more deeper, in 2.39 He said use buddhi to do yoga, so buddhi there is to understand the difference between matter and spirit to do yoga. Then in 10.10 Dadami buddhi Yogam Tam yena Mam Upayantite there the buddhi to do yoga is given by Krishna so the yoga given by Krishna is Bhakti Yoga, the buddhi that Krishna gives us is the intelligence to practice Bhakti and by both of these one can move towards transcendence, so here Krishna makes very clear in context that buddhi yoga is Bhakti Yoga, buddhi yoga is not a separate yoga, basically yoga is of four types only karma, gyana, dhayana and Bhakti and buddhi yoga is use of buddhi and what it refers to will be clear from the context.  Here Krishna is used it with upāśritya, taking shelter of Him so it is basically for Bhakti. 

mac-cittaḥ satataṁ bhava, always be consciousness of me in this way

so here Krishna is slowly but surely taking us to Bhakti Yoga and it will be more clear in future verses.

End of transcription.