This is a way of marking territory if they are rubbing the sides of their faces on the object. Cats have scent glands around their lips and on their chins. The facial pheromones that they mark with are signals of a happy or calm cat to other cats.
The part that simultaneously enables the axle to flex while transmitting the driving force is called a drive axle joint, and is also known as a constant velocity (or CV) joint. The CV Boot is a ribbed, rubber flexible boot that keeps water and dirt out of the joint and the special grease inside the joint.
Rear wheel drive cars and trucks have a universal joint in the driveshaft which accomplishes the same thing. CV joints are used in front wheel drive cars because the distance from where the shaft pivots to the wheels is much shorter, and therefore the joint has to pivot throuh more of an angle. Universal joints won't pivot as far as a CV joint will.