Ok, I see.
It seems that there is (thankfully) a protection circuit (see https://www.reddit.com/r/klippers/comments/xeywbj/comment/ir5y4yp/?utm_source=share&utm_medium=web2x&context=3)
The user reports that the fan turns on immediately when inserting the laser ribbon cable.
This makes sense to me, as most lasers I know are active-low, and without that, the laser would start burning as soon as you plug it in.
Does anyone have a detailed photo of the PCB, or even a layout diagram? There may be a switch that detects a plugged-in laser.