Simplex 2098 Test Station Question

I’ve seen a lot of these test stations on eBay and in person, and I’m wondering how they test detectors and what happens if one is set to test.

Photoelectric smoke detectors have an LED inside the detection chamber, that is pointed directly at the photo sensor, as well as another LED pointed at an angle to the sensor. When a test switch is activated, that LED is lit, sending a beam of light into the photo sensor, simulating smoke-like conditions and tripping the detector. Duct detectors (and sometimes regular smoke detectors) have a set of terminals where a remote test switch would be connected. When the test switch is activated, it lights the LED inside the detector to test the function of the photoelectric sensor.

(source: documentation, disassembly of a broken detector)

I am not exactly sure how an ionization smoke detector is tested, but I believe a small electric current is introduced in the detection chamber that changes the ionization of some of the air particles, and tripping the sensor. Could someone clarify?

For the Ionization detector, I think it requires a resistance of electric current between the plates, so I believe that switching it adds in a resistor to the plate monitoring circuit, which in turn makes it seem like there is smoke in the chamber, tripping the alarm. (correct me if I’m wrong)