In Neurology, there is a model called "The homonculus." This little man is representative of the proportion of neurons we have allocated to each part of the body. The large hands and lips represent how we have an enormous amount of sensitivity in these parts. A light graze of a feather would be felt instantly upon the hands, but on the back or chest, it might not be felt with the same pressure.
The key part I want to theorize here is as follows. These specific hand gestures may be used to trigger neural pathways to other parts of the brain. When the hands are held in these specific configurations, they could be activating other nearby areas of the brain. Or an even more plausible approach, is when you are told to meditate on a specific thought, feeling, or energy while holding your hands in a specific way, it conditions your neural pathways to link the two together. Once the linking becomes strongly conditioned, once the hands are placed into that same mudra, the same thoughts, feelings, and energies naturally arise without the need for a focused concentration. This is why they are so useful to meditation because individually, they become programs for the brain. A simple gesture activates all the deep insights of previous meditations by the links in the neural pathways. Research should be aimed in this direction to test the flexibility of this concept.
