That extra \ is just to tell the shell to ignore the line break, in other words to tell it you’re not done typing and to treat the next line as a continuation of the line. Basically, both the \ and the line break get thrown away.
By the way, if you really can’t get UEFI working and need to use legacy BIOS boot see my instructions here: Install HA on old laptop without UEFI - #20 by dbrand666