RAID Rebuild logging in VNX/Unity

Hi.

Can someone explain to me the underlying process of RAID rebuild logging in VNX or Unity. How does it help rebuild the RAID? The write data is stored in bitmap and when new disk is installed to degraded RAID - it's being rebuild with parity but new parity isn't calculated thanks to the rebuild log?

I have read the info in VNX and Unity documentation but can't wrap my head around the process.

Thanks.