The observation Wyatt Earp makes:

"There are two other points about the old-time method of using six-guns most effectively that do not seem to be generally known.
One is that the gun was not cocked with the ball of the thumb. As his gun was jerked into action, the old-timer closed the whole joint of his thumb over the hammer and the gun was cocked in that fashion. The soft flesh of the thumb ball might slip if a man’s hands were moist, and a slip was not to be chanced if humanly avoidable. This thumb-joint method was employed whether or not a man used the trigger for firing."

In Episode 22, Season 5, of The Life and Legend of Wyatt Earp named "Lets Hang Curly Bill" (originally aired Jan 26, 1960), Wyatt Earp defends Curly Bill Brocius in Court and saves his life by demonstrating how a professional gunfighter such as Curly Bill would cock his Colt 45 using the thumb-joint method vs the ball of his thumb. Curly Bill was drunk, shooting up Dodge. Marshall Fred White demands that Curly Bill turn over his gun. Curly Bill is in the act of drawing it from his holster to turn it over when Marshall White grabs Curly Bill's gun while Curly Bill's thumb joint is on the hammer, causing it to fire, and killing Fred White.

Deja vu on the argument from the TV show that just aired on Friday, May 8, on Encore Westerns.