I usually explain it this way.
Put a bowl on a table upside down.
Put your finger at the center of the bowl.
As you move out a little from center, you don't have to move your finger down much at all to remain in contact with the bowl. As you move farther out, you have to move your finger down more to stay in contact. AS you continue farther out, a little movement out means more movement down. This is the basic principle of expo