the SPR01 spring is the standard kit spring. You can bend these springs using the BS WORKS spring bender (shown in pictures) which SuperSrock will stock soon. Bending a normal/kit spring (often called normal or hard by users) allows you to increase rebound. When using a bent spring in the rear, you will gain exit steering. 

Having a collection of soft springs and normal springs, both bend and unbend is a handy tuning option to have. Please note; using a bent spring in the wrong situation will unload the rear too quickly, shifting weight to the front and causing a loose rear end. Bent springs are therefore suggested for tight tracks or high grip tracks only and not suited to fast, large flowing tracks such as NSWRCRCC or similar tracks where minimal weight shifting front to rear is preferred.