Kalavo, Tourist: Please note I'm talking about the Environmental Bleed Air Switch, not the Cabin Temp Mode Switch. The Environmental Bleed Switch controls the amount of bleed air volume entering the cabin. In the Low position the amount of bleed air entering the cabin is reduced significantly. Since compressing the air in the cabin will be reduced it will also result in a slightly lower cabin temperature. According to the FlightSafety manual "LOW must be selected for takeoff when the ambient air temperature is above 10 Degrees Celsius. NORMAL may be used for increased heating or if increased pressurization air flow is required.
Tourist: If you have a better understanding of what the switch does please enlighten me. The switch will increase bleed air entering the cabin resulting in increased heat. You see anytime you compress air the temperature will rise (all other factors being equal) (In very hot weather the P3 Heat Exchangers are not very effective).