| Presumably, you're talking about a slipper clutch. If the spider spring looses tension, you'd get less engine braking as the clutch would start to slip more easily under closed throttle. It shouldn't really affect forward motion, although that depends on the brand of clutch you're using to some extent.
If you do a search, I posted the measurements for checking the spider spring a while back. You easily measure the height of the spring when layed on a flat surface. | 
Spider in my clutch!!! 
Linear Mode
