I always thought that the finger thing was Lauras way of saying "follow your nose" or something. Like telling Coop to go by his instincts. I also think it has to do with drugs...
Re-reading Jospeh Conrad's Heart of Darkness, which I have always loved because of it's reverence for the wilderness and the mysterious dark force in/of them, I caught this gesture:
"I slapped him on the back and shouted, 'We shall have rivets!' He scrambled to his feet exclaiming 'No! Rivets!' as though he couldn't believe his ears.
Then in a low voice, 'You . . . eh?' I don't know why we behaved like lunatics.
I put my finger to the side of my nose and nodded mysteriously. '
Good for you!' he cried, snapped his fingers above his head, lifting one foot. I tried a jig.
We capered on the iron deck. A frightful clatter came out of that hulk, and the virgin forest on the other bank of the creek sent it back in a thundering roll upon the sleeping station.
It must have made some of the pilgrims sit up in their hovels.
A dark figure obscured the lighted doorway of the manager's hut, vanished, then, a second or so after, the doorway itself vanished too."
So many surreal moments (you never fund out who the dark figure is, btw) in that story, all the creeping madness...
Anyhow, here the signal seems to mean "I have the solution" or "I've got it taken care of." He never has to elaborate to the other man.