Icing the body when it is over-heated decreases body temperature rapidly through the process of:

When the body is overheated, icing it helps in decreasing body temperature rapidly through conduction. This process occurs when heat is transmitted from the warmer body to the colder ice, allowing the body to lose heat efficiently. Conductive heat transfer happens directly between two surfaces in contact, in this case, the body and the ice pack.

As the ice is in contact with the skin, it absorbs heat from the body, causing the temperature of the skin and the underlying tissues to drop. This method is particularly effective in cooling the body quickly because the ice has a much lower temperature than the body's heat, and the heat exchange occurs almost immediately.

Other processes like convection, radiation, and evaporation also play roles in heat transfer but are not the primary mechanisms at work when directly icing the body. Convection involves heat transfer through air or fluid movement, radiation involves the emission of heat as infrared waves, and evaporation relies on moisture turning into vapor, which is not directly applicable when using ice to cool the surface of the skin. Thus, conduction is the most relevant process in the context of applying ice to reduce body temperature rapidly.