What do gunslingers do with their hands?

You could say the gunslinger had his...

fingers/hands twitching over his guns _{(+more results for singular gun)}

twitch - to move jerkily or spasmodically (thefreedictionary.com)

I don't know of a formal term for this, but many seem to use hovering in situations like these.

For example:

with his hands hovering only inches over his gun grips (here)

and:

Duke takes his stance, hand hovering over the Colt (here)

and:

Their hands hover over their holsters. (here)

and:

She flexed her fingers, an old-style gunslinger hovering her hand over her holster (here)

and specifically in reference to a video game:

keep your hand hovering over your holster (here)

Hover, of course, doesn't capture the finger wiggle. But the finger wiggle might be so ingrained in the public consciousness that just using hover will suggest it to those familiar with the imagery.

I was also thinking fingering might work, but it suggests actually touching the gun and it is often used in conjunction with nervously, as in "He was nervously fingering the gun in his holster".

Another option is flutter, which has the same benefits as hover, while also suggesting movement.

A final option is to describe the fingers as undulating over the holstered gun, where undulating means "move with a smooth wavelike motion" (Google).