> Fair enough, but the burning question is HOW?

Here are a few rudimentary examples which should get you started:

program |&   # start program as co-process

read -p inputline   # read line of data from co-process into variable
                    # inputline

print -p "foo bar"   # send data "foo bar" to co-process

anotherprogram <&p >&p # run another program, connecting its
                        # standard input and standard output to co-process

exec 3<&p 4>&p  # connect file descriptor 3 to input from co-process and
                 # file descriptor 4 to output to co-process by default

read -u3 inputline   # read data from co-process, this time through file
                     # descriptor 3 (comparable syntax for print as well)

program2 |&   # now that the prior co-process has been connected to file
              # descriptors, we can start a second co-process without
              # the two interfering with each other

Eric Amick