In general, when you apply carb heat you exchange the cold air from one source with the less dense warm air from another source, thus resulting in a reduction in power ( manifold pressure), This is most obvious with a fixed pitch propellor: i.e. a loss of RPM owing to the reduction of MP. The constant speed propellor will, of course, with a reduction of power then fine off maintaining the RPM. Carb heat enrichens the fuel/air ratio and so there is the possibility that for a brief period only on returning to cold air there is a temporary boost to the mixture (as with all instruments there can be a marked lag though), but
I suspect that you are running the engine over lean and, if so, adding carb heat will mean that you are returning the mixture to a more correct fuel/air ratio so the increase. To achieve best lean the process is often described as: first leaning off until the RPM drops and then to go back towards rich to find peak rpm.