Here we are dealing specifically with the vowel system, but a similar model could be applied to a change such as Grimm's Law.
Language change is to be understood as a change in the make-up of grammars.
Since grammars are conceived of as a specific ranking of universal constraints in OT, it follows that language change must be a re-ranking of those universal constraints. Actual historical data support our argument that the GVS can be broken down into 3 main stages. Stage II is the most active stage of the change, when the bulk of the alternations are shown to take place synchronically. The chain-shift is modeled with the help of local conjunction and cooccurrence constraints penalizing, for instance, the long mid lax vowels. The novelty of the proposal consists in singling out the mid lax vowels as the initiators of the change.