./bar < <( ./foo )
cat < <(echo "hello there!")
To understand how it works, consider parts of the script separately.
cat < /path/to/file will read the file
/path/to/file and pipe it as stdin to
<(echo "hello there!") means to execute the command and attach the stdout to a file descriptor like
/dev/fd/65. The result of the whole expression is a text like
/dev/fd/65, and a command run in parallel and feeding that file descriptor.
Now, combined, the script will run the right command, pipe it to a file descriptor, convert that file descriptor to stdin for the left command, and execute the left command.
There is no overhead that I'd know of, it's exactly the same as
a | b, just syntactic sugar.