The PSR FLEX design is a true progressive spring that when loaded increases its spring rate and when unloaded decreases in spring rate, this means it acts like a HD coil when more weight is added or pushes into its travel but when the suspension is on its rebound stroke it softens to allow better ride quality especially important when offroading.
A feature of the PSR FLEX is a longer free height meaning you can run a longer shock absorber for more articulation and off road performance before the spring becomes untrapped. There is a lot of poor information in the marketplace about flexi coils but generally what is supplied from other companies is a coil that is a super soft spring that is linear rate to allow for a longer free length for spring trapment. This then translates to a super soft coil with limited capacity to accept accessories and control body roll.
