It is a horribly complicated PWM regulator which chops the field current on/off. What is this thing supposed to do that a $20 OEM replacement bought over the counter at PEP Boys wouldn't do????
Oh, I went and read the description. I see now that it is supposed to limit the initial charging current in addition to regulating the charging voltage later in the charging cycle. It is a dumb circuit, in as much it doesn't actually measure the current flowing into the battery; it just does an open-loop reduction of the available charging current.
If you are using flooded-cell batteries, the current limiting is unnecessary, they will take anything an automotive alternator can put out; with AGMs, current limiting might be beneficial, depends on your batteries, and how much current the alternator can put out.