it's just a slightly more extreme form of the glove-and-hand principle (versus lock and key). Of course even this turns out to be a spectrum. there are enzymes/protein that are more lock-and-key, and proteins that interact very heavily. One extreme is insulin receptor (which totally mangles insulin out of shape on binding). IIRC, another interesting one is antibody evolution, as B cells mature they go from producing antibodies that are gloves-in-hand to antibodies that are lock-and-key, as measurable using IR spectroscopy: https://www.pnas.org/content/103/37/13722.short
