When the chirality of the product is an autocatalyst to its own chirality. So if your reaction starts off with a little more left than right due to molecular randomness, the left is an autocatalyst which means there is a runaway process and you end up at the end of the reaction with a lot more left than right.
If you repeat the reaction from scratch, it is random if you end up with a left product or a right product. But the product of your reaction is not a 50-50 mix left-right! I.e. you created asymmetry in your solution from purely symmetric solvents.