What is "convexityDefects" in opencv ?

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Hi , i am implementing a hand recognition system using opencv , i found that i must be using "convexityDefects" : what i have understand from this page documentation : convexityDefects

that it is used to count fingers of the hand hull by determining the minimum distance between the two points on the hand .... but i still don't know exactly how this procedure is done , can anybody give me more explannation about it also what algorithm is used for hand gesture recognition in opencv

Any help will be appreciated , thanks