Alter command to extract iso content , tar cf - . | (cd /tmp/testiso_temp; tar xfp -)

I have mounted an bootable iso image onto /mnt/testiso
Code:$df -H
/dev/loop12 17M 17M 0 100% /mnt/testiso

$pwd
/mnt/testiso

As you can see the content of the iso.
$ ls
boot boot.catalog efi.img mach_kernel System

$ tar cf - . | (cd /tmp/testiso_temp; tar xfp -)this is used to extract the content of the iso to a temporary folder (/tmp/testiso_temp).

Can you please explain to me how this tar with all those options extract the content of the mounted iso to a new file for me to modify ?

I tried to understand the tar options, but just can't get it.
Code:$ tar cf - /mnt/testiso/ | (cd /tmp/testiso_temp; tar xfp -)when i tried to change the code as above, the output of it is weird.. instead of it extract the files to /tmp/testiso_temp, it even move the whole /mnt/testiso to /tmp/testiso_temp...
why ?

I just want to alter the code so that i can insert iso path instead of have to be in the directory itself..

Thx
Thx