This paper describes how to use implicit methods for simlulating cloth
rather than the explicit methods that were the dominant ones back then.
While explicit methods require small time steps to require numerical
stability, implicit methods alow larger steps that can greatly improve
performance. The method they describe doesn't introduce additional
penalty terms  in energy or add a Lagrange-multiplier force.
Instead it uses the fact that the inverse mass matrix can be exploited
to enforce constraints and use modified conjugate gradient method to
to deal with the fact that CG methods require symetric meatrices.