Redirections are implemented via the dup family of system functions.
dup is short for duplication and when you do e.g.:
you duplicate (
dup2 ) filedescritor 2 onto filedescriptor 3, possibly closing filedescriptor 3 if it's already open (which won't do a thing to your parent process, because this happens in a
forked off child (if it does not (redirections on shell functions in certain contexts), the shell will make it look as if it did)).
When you do:
someFile on a new file descriptor (that's what the
open syscall normally does) and then it'll
dup2 that filedescriptor onto 1.
What the manual says is that if one of the special dev files listed takes the place of
someFile, the shell will skip the
open-on-a-new-fd step and instead go directly to
dup2ing the matching filedescriptor (i.e., 1 for /dev/stdout, etc.) onto the target (filedescriptor on the left side of the redirection).