Yoga Sutras 3.16
Patanjali, explained by Swami Vivekananda
परिणामत्रयसंयमादतीतानागतज्ञानम् ॥
pariṇāmatrayasaṃyamādatītānāgatajñānam
By making Samyama on the three sorts of changes comes the knowledge of past and future.
In plain words: Samyama is deep, focused attention held on one thing until the mind becomes one with it. When a yogi does Samyama on the three kinds of change (in form, time and state), he comes to know the past and the future.
Swami Vivekananda explains
We must not lose sight of the first definition of Samyama. When the mind has attained to that state when it identifies itself with the internal impression of the object, leaving the external, and when, by long practice, that is retained by the mind and the mind can get into that state in a moment, that is Samyama. If a man in that state wants to know the past and future, he has to make a Samyama on the changes in the Samskaras (III.13). Some are working now at present, some have worked out, and some are waiting to work. So by making a Samyama on these he knows the past and future.