The "big picture" behind tcharge is to try and more accurately predict the temperature of air entering the cylinder, as this is what matters more than the temperature of air wherever the intake air temperature sensor is or the temperature of the coolant sensor.
The broad principles here are that slower moving, less dense air has more of a chance to soak up heat from the intake manifold and cylinder head compared with high-flowrate air where the higher velocity gives less time for heat transfer combined with a larger air mass to dilute the effects of any heat absorbed.
With zero airflow, the temperature of air is assumed to be the temperature of engine coolant. With infinite airflow, the temperature is assumed to be the temperature indicated by the air temperature sensor. In the middle, you have a tcharge model. The idea here is to do a better job of handling temperature variations than is possible with only air temp and coolant temp enrichments by more precisely modeling what is actually happening. This type of model has been common in OE engine management for at least 15-20 years.
I'll let someone else chime in on how the rusEFI settings relate to this framework.
