A lot of 912s, including mine have a water jacket around the throat. Initially I was still in 'carb heat ' mind set thinking that's a waste of power because it's always on. But there is virtually no difference in power because all it is doing is keeping the throat surface warm, preventing icing. Great idea, after all that's the bit you don't want iced up, not the air/fuel mix.
Note, there is also an electrical version.