Or HaChaim on Genesis 30:40/אור החיים על בראשית ל׳:מ׳

1 פני הצאן אל עקוד, the flocks facing the ankle-striped, etc. Perhaps the reason that only the ankle-striped ones are mentioned here is that the sheep would react more rapidly at the sight of that colour pattern than towards the striped or speckled ones.

2וישת לו עדרים, he set aside droves for himself. This was designed so that these flocks should never look at any other varieties than the ones with the skin pattern Jacob had chosen. In this way they would automatically give birth to young ones of that same skin pattern. If that were not so Jacob would have to give to Laban all those young sheep or goats which were not speckled or striped.