Hard to describe with words. The "knub" is called a clip horn, even though it is not a horn. A clip can be drawn at the toe or side(s) of the shoe. To begin, the shoe is held at about a 45 degree angle off vertical, foot surface down, ground surface up. A small portion of the foot surface near the outer edge of the shoe is made to contact the far semi-circular edge of the clip horn. Working hot, that portion of the shoe is hammered back into the clip horn. As the hammering continues, the shoe is lowered bit by bit until it is no longer at 45, but plumb, ground surface away from you, foot surface toward you. If all goes well, you will be drawing a relatively thin projection, the clip, over the top of the clip horn. When the shoe is nailed onto the hoof, the clips are visible and are cold hammered toward the hoof wall to conform to the wall angle. Clips do not help hold the shoe on the foot. They keep the shoe from shifting on the foot, especially used on athletic horses, such a hunters, jumpers, and game horses.
After the clip is drawn, an inspection of the shoe's foot surface will show a slight crescent shaped depression which is where some of material came from to form the clip. Lots of farriers did not like this depression; they termed it "gutting out the foot surface." Therefore, most farriers drew their clips off the straight side of the anvil with the ball end of a ball peen hammer or a similar hammer of their liking. This preserved more of the foot surface.