Lever Point

Lever points in a complex adaptive system are points at which a small effort can produce a desired, directed effect.

John H. Holland writes about them (see here)

"Almost every complex adaptive system that we know of has lever points. These are great things if you can find them. A lever point allows you to make big, directed changes for a relatively small effort. A vaccine is a good example of a lever point. An immune system is terribly complex. But we do know that for some diseases, we can make a small injection and train the immune system to resist a disease for the rest of its existence — much longer than the life span of any blood cell in your body."

see also butterfly effect, path dependence, frozen accident