The model explains why objects out of focus looks blurred at a small diaphragm value.

If the object is outside the focal line, then on the opposite side of the lens the rays, which are going from object point, will cross outside the plane of the sensor (matrix or film). So the rays will be displayed not as a point on the sensor, but as a spot. That spot is formed from all rays from the point of the object passing through the aperture of diaphragm. The combination of such light spots forms a blurred image of the object. With a large diaphragm value, the beam spot is smaller — so the object on photo looks sharper.

Change diaphragm value and move test objects to study how light projection to sensor is changing and forming image.