How is the password file accessed when it's on an ASM diskgroup and required to connect to ASM? When a RAC is turned on, it will try to accessing the OCR registry which is located on ASM.
ASM also holds the asm password file.
However to open ASM, it needs to access the password file.

If the password file is missing or corrupted etc... when turning on the CRS stack, this happens:

2016-01-12 12:00:51.317680*:kgfo.c@1115: kgfoSaveError: ignoring existing error:
ORA-01017: invalid username/password; logon denied
ORA-15077: could not locate ASM instance serving a required diskgroup


Oracle's solution to this is the following:

You can make a backup of the Oracle ASM password file, and if the Oracle ASM 
password file is lost or the disk group becomes inaccessible, then you can 
restore the backup password file. 


Even if ASM is working, how can it access the password file which is inside ASM if it needs a password to access ASM?
ASM metadata describes the location of the password file on the ASM disk. With that information, even you can read it directly from the disk, with the help of kfed (which is installed with the Oracle binaries) and OS specific tools for reading the disk directly, for example dd, without using ASM. Same goes for the SPFILE (since 11g).
More about this topic:
ASM spfile in a disk group
The ASM password directory
And you do not need a password file to start the ASM instance, that is needed only for remote connections.