Separating part of the flour to be used for the Matzah:
Some of the great Poskim accustomed their generations to separate part of the flour designated for the Matzah and to cook it and eat it before Pesach. [However] it is best to give it to paupers.
The reason for this is: By doing so, one has kashered the flour from a suspicion that perhaps it had some Chametz flour mixed in it, as now even if it did have, we can [Halachically] assume that it was all included in the flour eaten prior to Pesach.
An obligation or stringency? Nevertheless this custom is a mere stringency, as from the letter of the law we do not assume that Kosher foods [non-Chametz flour] has a mixture of non-kosher [Chametz flour].
 lit. Tov