[litew8]

The top hose should be empty when the thermostat is closed. When the thermostat OPENS, the pressure maintained/created by the water pump forces the heated coolant to pass by the thermostat (via engine) and enters into the radiator through the top hose -thereby causing the cooled coolant previously in the radiator to enter the engine via the bottom hose. When that cool coolant enters the engine, the thermostat recognizes and closes - allowing the newly heated coolant in the radiator to cool. Rinse, repeat.