A model for interaction of a protein with a ligand in which the protein binds preferentially with one of several conformers of the ligand; especially enzyme action in which the substrate is proposed to bind to a conformation of the enzyme that produces a strain on the substrate, focused on the bond that must be broken to effect catalysis; also known as the handshake model, and contrasted with the lock and key theory, in which both protein and ligand are imagined to be inflexible.