When the sheep mated ¹  in front of the branches, they ²  gave birth to young that were streaked or speckled or spotted.

But Jacob took fresh-cut branches from poplar, almond, and plane trees. He made white streaks by peeling them, making the white inner wood in the branches visible.

Then he set up the peeled branches in all the watering troughs where the flocks came to drink. He set up the branches in front of the flocks when they were in heat and came to drink. ¹ 

When the stronger females were in heat, ¹  Jacob would set up the branches in the troughs in front of the flock, so they would mate near the branches.