You actually have 2 springs, the fork spring and a "air spring" from the trapped air space in the fork. This air spring as the fork compresses is very progressive in its action.i f your fork rides low in the stroke because of too soft a fork spring you will be riding in the firmer part of the air spring. If the correct stiffer fork spring is installed the fork will sit up higher in the stroke and consequently sit and operate in the much softer part of the air spring. This will make the fork action plusher as well as help stability and cornering . This is also why you do not want to run air pressure in your forks if so equipped as this exaggerates the progressive nature of the air spring. If replacing fork springs you want a "straight rate" not a progressive rate as you do not want to add any more to the progressive nature of the air spring.