This is the part of the rulebook that caused so much controversy when new rules were being developed and applied in the mid-2000s: the spring-like effect. High-COR drivers, low-COR drivers, legal drivers, illegal drivers. And COR ... what exactly is "coefficient of restitution" anyway?
What you need to know is that just as there are non-conforming golf balls, there are non-conforming drivers. And the USGA and R&A maintain lists of non-conforming drivers.
If your driver is on that list, it means that the driver exceeds the limit of COR put in place by golf's governing bodies. According to the USGA and R&A, the COR of a driver may not exceed a measurement of .830 (above that, they say, and the driver begins to act as a spring) in any competitions played under the Rules of Golf, including handicap rounds.
Ready to view the lists of non-conforming drivers? These links take you to the lists on the USGA and R&A websites, respectively:
Both governing bodies also publish lists of drivers that do conform to the rules on springlike effect.