Well, given that this is an NW mod and not a Warband mod, that's something I'm sure would be exploited fully. However, I'm pretty sure there's no toggling of function with shields. Looking at the shields in the screenshots, it doesn't seem likely for it to be included at all, given that it's not a Scutum (the shield that goes hand-in-hand with the formation), and instead, a Parma-like Republic-styled shield.