A group of cloaked smugglers and thieves use an unspoken language of hand signals, like ASL. However, I was imagining they use finger digits, the position of the thumb, and the rotation of the hand instead of hand motions (that may require whole arm movements), though this is just an idea; I'm open to others.
The goal is to make the signals as quick as possible and with as little effort as possible, so that if they get in tight situations they can communicate efficiently. Under these constraints, what are the bare minimum concepts required for adequately communicating? By this I mean either having very few things that one is able to say in the language (and therefore my question is what would those be), or possibly having a set of very vague and very precise concepts that work together (and therefore what would those be).
Because the signals are just a combination of finger positions it could be easy to mix up signs and misunderstand. Depending on the answer to the first question, there may be enough concepts assigned to each finger combination that misunderstandings would be caused. If this is true, what should be changed in order to quickly, with little movement, convey the same ideas?