I would like to know by what method the RS reduces output whenever it needs to, like when battery charging reaches absorption phase and there are not enough loads to use available solar production. Are there any data that can be taken from it as a "trigger" when it does so?
Absorbtion and Float are constant Voltage
It simply doesn't take the power from the solar panels. You can think of it as simply letting the solar panel go open-circuit for a tiny fraction of the time before reconnecting it. It's more complicated than that, but that's the gist of what's going on. You can have a solar panel in full sun, with nothing connected to its output, and it will thus generate no power.
If it's not under external control from the BMS (or whatever is managing the power) the MPPT watches its output voltage, and watches it to produce the programmed voltage curve.
