What Bill Keith was doing was not twisting the "regular" tuning machines on the banjo. Banjos can be fitted with some kind of extra lever to sort of "toggle" the string back and forth between two preset notes. I'm pretty sure they're installed in addition to the regular tuning machines, not instead of them, and I think they may have been put on only two of the strings, although I'm less sure of that.

You can notice that each retuned string always goes back precisely into correct tune. That wouldn't be possible if the player were actually retuning the strings at that speed. The tuning at which you start the tune is basically "locked in," and the supplementary device lets you stretch the string to a different note (at a precise interval higher, or maybe lower) and then let it return to its exact original tension.

You used to see this "trick" performed all the time back in the folk-revival heyday, by players with appropriately-equipped banjos. I think those devices were originated by Earl Scruggs, but ~ not being a banjo player, just an observer ~ I don't know for sure.

Use of those whatchamacallits was an absolutely mandatory feature of playing one of the most popular banjo-virtuosou tour-de-foprce numbers, "Orange Blossom Special."