With a broadcast spreader, the spreading material (seeds, fertiliser etc) is dropped into a hopper, generally constructed from a tough, corrosion proof material like polyurethane, occasionally galvanised or stainless steel.
As the garden spreader is pushed along or towed behind, gravity and motion pulls the material down and it falls through an aperture onto a spinning plate, which is positioned at an equal distance between the wheels
The spinning movement of the broadcast plate is controlled by the wheels of the garden spreader * and the spin of the plate spreads the fertiliser or seeds in a wide pattern
* N.B. On some large commercial agricultural spreaders this operation is motorised