In the film "The Sting", how did the con men signal each other?

In the 1973 film "The Sting", the con men signal each other by tapping their noses. The men use this gesture to specifically acknowledge when their part of the scam/scheme is completed. This film stars Robert Redford and Paul Newman (Butch Cassidy and Sundance Kid). They are reunited with director George Roy Hill from "Butch Cassidy and Sundance Kid" (1969).

Set in 1930s Chicago, the film, "The Sting", opens up with novice con Johnny Hooker and his mentor, Luther, as they pull off their latest sting. Unfortunately, they've bitten off more than they can chew this time. Their latest victim has connections to Big Bad racketeer Doyle Lonnegan. When Lonnegan's goons rub out Luther and put a hit on Hooker's head, Hooker seeks out his mentor's friend, Henry Gondorff, a master of the "big con". Assembling their team, Hooker and Gondorff prepare to scam the big one out of Lonnegan. Just to complicate things, they've got to make sure he never finds out he's been conned, otherwise he'll put hits on all of them.