Never heard of the "double-knock" but I like the poker story. I have also seen
placing an upside down glass in front of the guest a sign that someone bought
their next drink - sort of as a reminder.
But beware - many older or more experienced bar drinkers just try to use this
to intimidate you into buying them a freebie. It's your bar - you make the
rules. If you want to "double-knock" or ring bells or whatever, you should, as
the previous poster implied, TELL people what you are doing. It is just good
business. It can be very annoying to people who are visiting a place for the
first time magically see free drinks appear all around them and not know the
system. I worked one establishment that rang a bell for significant tips. One
night three businessmen came in, and when they heard the bell said, "oh, well,
where WE come from, the bell means everyone at the bar gets a free drink!"
They harassed the bartender so much they finally had to get the boss to tell
them to shut up.
Don't even try to catch me making comments on "my ethics."