First things first, a rooster does not impregnate a hen. Hens do not get pregnant, they lay eggs which can be fertile or not.
Eggs are fertilized when a rooster mates with a hen. It’s not sex as we think of it; roosters do not have penises and hens do not have vaginas in the same way that mammals do. Both sexes have an opening called a cloaca that is used for mating and excrement. Eggs are also laid through the cloaca.
When a rooster initiates a mating, he will circle around the hen with one wing dropped. In an ideal world, she will crouch and flatten her back for him and hold still.
Holding the feathers on her head for balance, he will mount her. Her tail will be lifted and his will be lowered so they can touch cloacas. This is called the “cloacal kiss.” Sperm is passed from the rooster to the hen.
The whole process takes only seconds. If the hen didn’t crouch for the rooster, he’ll often grab her and mate her anyway. A rooster will mate a hen multiple times a day.
Once the sperm is in the hen’s body, it is held in the oviduct. As the yolk leaves the ovary and enters the oviduct, the sperm joins with the ova and fertilization occurs.
The egg then will not develop until certain conditions are met. The most important of these is that the egg is at approx. 99 degrees Fahrenheit for over 24 hours.