Through direct perception of subliminal impressions, one gains knowledge of previous lives.
When one observes the contents of the mind purely as a witness, the subconscious will start to bubble up memory impressions. All life experiences, both internal and external, leave an impression on the mind which can be later called up through memory. So by cognitive absorption on the fluctuations of the mind and tracing them back to their origin (without getting lost in fantasy), it is possible through mastery of this Samyama to relive the memories of past lives.
The twin wings of Yoga are Viveka (discrimination between the real and unreal) and Vairagya (dispassion towards the ephemeral). The practice of seeking to know past lives can lead to attachments at a more subtle level and at worst delusional fantasies of ego constructed self-importance.
Techniques and their outcomes should only be judged useful on the basis of greater understanding of self and the universe with the ever greater aim of transcendence.