What happened when Yakov placed the various peeled sticks in the troughs of water when the sheep and goats came to drink?
Rashi #1: When the females saw the sticks, they were startled and recoiled 1 , and the males in back of them mated with them. 2 and they conceived babies similar to what they saw (white spots and blotches). 3
Rashi #2: A miracle occurred and the water turned into semen, and they conceived without any contact with a male. 4
Gur Aryeh: One who suddenly sees a strange matter recoils.
Rashbam - the word "va'Yechamnah" (and they became hot)" is masculine and feminine. Tosfos ha'Shalem (4) - this teaches that the female were heated through the males.
In verse 39, Rashbam points out that "va'Yechemu" is totally masculine. Perhaps he explains that both opinions in the Midrash are true - some females became pregnant through males, and others through water alone (PF).
Why does it say "l'Nochach ha'Tzon"?
Malbim: The color of other animals would make an impression on flock animals, but the sticks would not. The animals came to drink from both sides of the trough. The sticks were in the middle, and it seemed to the animals that the colors on the sticks are on the animals on the other side.