a progressive coil spring is one that is wound so that the spacings are progressivly larger. Each section / turn of it is linear - but as you compress it more and more coils bind so you have less coils active so the total effective rate goes up.
Air springs will always be progressive but depending on the ratio between the volume at the start of the stroke and the end it can be very slight progression or great.
another experiment. Take a set of old school 'zocchi open bath forks. take all the oil out - you get an almost linear spring. The more oil you add the more progressive it becomes as you alter the ratio of the volume at the start of the stroke to the end of the stroke until you reach a ponint where you cannot get full travel as you have zero air gap left thus infiniate pressure before you get full travel.
