Why is full carb heat recommended when reducing power below normal setting?
When you close the throttle for descent, the airflow is constricted.  This constriction is going to cool the air more than it is during cruise configuration, and may increase icing.

From This AOPA document:


Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the. throttle valve.