It happens after each life is over. One is left with nothing in terms of memory information and reference, when one takes an embryo. A yogi can use a condition like that to ponder as to what the core-self really is when it is shorn of its adjuncts and their physical props like the brain.

Ramana (Raa-muh-nuh) Maharshi was the person who stressed that the core-self was a neutral reality with nothing but a floating identity which has no coloration or tendency, when the self is shorn of the adjuncts. He spend his life anchored in that state of neutrality.

http://realization.org/p/ramana/ramana.html