Genesis 30:37-43 New International Version (NIV)
37 Jacob, however, took fresh-cut branches from poplar, almond and plane trees and made white stripes on them by peeling the bark and exposing the white inner wood of the branches. 38 Then he placed the peeled branches in all the watering troughs, so that they would be directly in front of the flocks when they came to drink. When the flocks were in heat and came to drink, 39 they mated in front of the branches. And they bore young that were streaked or speckled or spotted.40 Jacob set apart the young of the flock by themselves, but made the rest face the streaked and dark-colored animals that belonged to Laban. Thus he made separate flocks for himself and did not put them with Laban’s animals.41 Whenever the stronger females were in heat, Jacob would place the branches in the troughs in front of the animals so they would mate near the branches, 42 but if the animals were weak, he would not place them there. So the weak animals went to Laban and the strong ones to Jacob. 43 In this way the man grew exceedingly prosperous and came to own large flocks, and female and male servants, and camels and donkeys.
We are not sure as to how Jacob’s method worked. It is unclear what this method was, but it is said that there was a belief among herdsman that vivid impressions at mating time influenced the offspring. Jacob was using the method of selective breeding. He also had God’s promise that God would provide for him. God’s promise was the main reason that Jacob’s flocks increased. You can always trust God’s promises.