The fulcrum is a moveable wheel that sits underneath the mid-point of a springboard diving board, allowing a diver to adjust the amount of spring they receive. The rear of the diving board is fixed to the diving stand, so as weight is applied to the tip end of the board, the board will flex from the point at which the fulcrum sits.
As the fulcrum is moved backward, the diver will receive more spring. As the fulcrum is moved forward, the board will flex less and the diver will receive less spring.
Divers learn through practice how to adjust the wheel to get the maximum amount of spring depending on their weight, and whether they are using a forward or backward takeoff.