I watched Kevin Fore and other clawhammer players last night on a TV program. There was a whole lot of right hand finger-flickin going on. And it made sense to me, as I watched it. Fore's hand barely moved up and down the strings, and he was hitting those strings with great accuracy, with his flicking finger. That sounds almost vulgar, but I didn't intend it as a pun. And that got me to thinking -- here I am, trying to adhere to the rules, stiff index finger and all, hand and arm moving up and down, swatting at the strings and hoping I hit one. I think there's a lot of wasted motion there, which can create a lot of inaccuracy, and that may not be what some of us need. It may be that the good banjo players don't play by the rules established by someone else, they take their own physical constraints and abilities into consideration. In Kevin Fore's case, he even flicks with both the index and the middle finger. I could plainly see it, the motion was clear.
Maybe the "no-flick" rule comes from wanting to get max volume, for playing in groups. Is that possible? If true, and one doesn't play in groups, maybe the rule doesn't mean much. It most likely, though, came from people trying to help other people learn, and they just naturally showed them their own way of playing, until it finally became a sort of sacred rule.