According to self-professed movie expert Doug Spice, this theory aims to explain that the presence of a one finger in a Harrison Ford movie is indicative of the movie's quality. He bases his theory on the "Indiana Jones" trilogy. For example, "Raiders of the Lost Ark" and "Last Crusade" are probably the best films of the trilogy because they feature Ford waving one finger prominently. "Temple of Doom", however, lacks the one finger waving and, therefore, wasn't as good in quality.
I'm not sure how this theorem holds up when used to compare the "Star Wars" trilogy because Ford seems to be waving that finger a lot in all three movies. It didn't seem to make "Return of the Jedi" any better, though.
A derivative of the theory explains Chow Yun Fat's finger waving in films where his character dies...it's called Dan's Theory of the Digit of Doom.