From x.org old docs:
On nvidia card, it's 2-3% single 1.6Ghz Sandy Bridge core (when uncomposited, of course)5.15. Option ``backingstore''
This is not specifically a driver option. It is used to enable the server's support for backing store, a mechanism by which pixel data for occluded window regions is remembered by the server thereby alleviating the need to send expose events to X clients when the data needs to be redisplayed.