Your knees bend because the force pulling on your upper half pulls your body down, and that causes your knees to bend. Your knees act like a spring and a cushion of sorts, so that the force on your lower legs isn't as intense.
You should feel a harder impact if you don't bend your knees. Keeping your knees unbent means your muscles are more tense and that adds more wear and tear on your body. There isn't a cushion this time to slow down the force of the blow, and the stopping time is much more quick.
The diagram will consist of the person, which you can draw as a rectangle, and arrows to show the various forces applied to the rectangle. The two main forces are the force of gravity pulling down and the normal force of the ground pushing up. There won't be any arrows pointing left or right.
The normal force pushes up on your body. This force is the ground pushing up, as your diagram would show. The normal force is why people and objects don't fall through the floor. The normal force is the result of molecules repelling one another so objects don't go through each other.
When your body hits the ground, specifically when your feet hit the ground, there is a net upward force of the ground pushing up. This causes the bounce, so to speak, that happens. The bounce is very subtle and sometimes easy to overlook. Using a bouncy ball, the bounce is very clear. With other objects, you may need to drop them from a large height. Try dropping a piece of plastic from the height of a counter top or table top, and you should see the plastic bounce up as it hits the ground. This is the normal force counteracting against the force of gravity pulling down. In terms of your feet/legs, the normal force pushes up and that's why you feel the sensation of landing (the height of the fall depends on if the sensation is painful or not).
