Bandha-karana-saithilyat pracara-samvedanat ca cittasya para-sariravesah.
Bandha = bondage; karana = cause; saithilyat = on loosening; pracara = channels; Samvedanat = from knowledge of; ca = and; cittasya = of the mind; para = another's; Sarira = body; avesah = entrance.
The mind (of a yogi) can enter another's body on loosening the cause of bondage and from the knowledge of channels (passages).
This sutra is about one of the well-known siddhis in yogic tradition called Para-kaya-pravesa (entering another person's body). People in India are familiar with many anecdotes about this Siddhi. This, however, is not to be mistaken with the episodes where it is believed that lower souls possess somebody's body by force. Here, a yogi, a master of his senses, enters other one's for selfless purpose, for some good of the society. He may either enter a dead body before it is decomposed or the body of his disciple who has willingly withdrawn from his body temporarily for his master after learning the required techniques. Till the purpose of the great master is over, the consciousness of the disciple remains at a higher plane.
First step to acquire this siddhi is to loosen the bond that ties the consciousness to the body. And the cause of that bond is stock of all karmas; karmas that are created due to Raga-dvesa (like and dis-likes), abhinivesa (clinging to life) etc klesas (called pancha-klesas) that make us identify with the body. In fact, the very cause of our birth is past karmas. By overcoming klesas yogi can dilute the effect of karmas and the bondage to the body can be relaxed.
And the second step is the knowledge of that particular passage, that channel (nadi) through which yogi's citta can enter the alien body. Various nadis are vehicles for prana etc. According to occult tradition it is citta-vaha-nadi that is vehicle for citta through which a master can come and go out of other person's body.
It is well-known that the great Acharya Shankara, when confronted with question regarding the life of a house-holder in a debate, enters the body of a king who had just died and goes through required experiences.