: is the null command. Kind of like /bin/true. So : followed by anything exists simply to perform expansion on something. If you fully understand what that mean

: is the null command. Kind of like /bin/true. So : followed by anything exists simply to perform expansion on something. If you fully understand what that means, then you will understand this: $_ is simply “whatever the last argument to the previous command expanded to”. So : is the previous command and the ${1… expanded value is now ${_…. Because $_ is used in the expansion of itself, it is the same value, because the command has not yet completed, which would (re) set $_. So use of a thing ($_) in expanding the same thing ($_) is perfectly fine until after that (null) command (:) runs. You see that the final $_ is used standalone. Hope this helps.