Battery Backup Write Cache, Flash Backed Write Cache
It is all about saving your data should the power accidently go out.
FBWC is a flash based cache module that does not have the battery limitation of how long it can retain what is written to the module.
You can find FBWC architecture via this whitepaper. HP Smart Array Controller technology – Technology Brief, January 2010. See pages 20 – 23 in: