DY,
The sensor has to detect the pulse, which then causes the light to turn on. (or off)
But it is not a constant stream of light. If it were, it would be simple, block the light the switch does one operation, unblock the light, however this switch does something else.
The switch is supposed to be working with a random pulse of light. So the only thing that can activate the switch would be the light pulse hitting the sensor. Which can only happen after the plastic of the switch is moved. But by definition, it would seem that light NOT hitting the sensor cannot do anything because the light is not constantly being emitted. If it worked that way, the switch would then become like a blinker. It couldn’t even be set to change after a defined amount of time, because the emissions/pulses are supposed to be random. So it would seem that switch would only be able to effectuate one change over a Shabbos.