What happens during Disk Initialisation?

I'd like to know what "Disk initialisation" actually does from the Dell virtual disk management screen (i.e. the RAID configuration).

When I add a physical disk to a logical drive, it gives me the option to "initialise disk" but what happens? When I perform it, it says I will lose all data, so does it perform a format?

Also, when I carry it out there's no progress bar, or any indication that anything has changed - so if it is doing something it apparently happens instantly. Is this normal?

Thanks.