As a long-standing puzzle, experimental observations reveal faster organophosphine dissociation in the olefin metathesis by Grubbs's first-generation precatalyst (Gen I) than by the second-generation precatalyst (Gen II), but Gen I shows less catalytic activity. Here we show by electronic structure calculations with the M06-L density functional that carbene rotamer energetic effects are responsible for the inverse relation between organophosphine dissociation rate and catalytic activity. The carbene rotamer acts as a toggle switch, triggering the dissociative mechanism that produces the active catalyst. The slower catalyst production in Gen II as compared to Gen I is not a pure electronic effect but results from rotameric coupling to the dissociation coordinate speeding up Gen I dissociation more than Gen II dissociation. If organophosphine dissociation were to occur with fixed rotamer orientation, Gen II would be produced faster than Gen I, as originally expected. The rotameric energetics also contributes to the higher catalytic activity of the Gen II catalyst.