Alter Solenoid Stroke
The force of the solenoid varies over the length of the stroke, reaching a maximum at the end of the stroke, when it is fully activated. The minimum force occurs when the solenoid is just starting to move from the deactivated state.

"Preload" refers to partially compressing the solenoid's return spring by mechanically preventing the solenoid from reaching its fully deactivated position. If you do this, the initial force when it starts to move will be higher, which is what the chart reflects.