According to some of the Switch 2 rumors reported on by The Verge, pressing that button actuates a pin that will push the Joy-Cons away from the console, and in the video, you can see on the ...

When the momentary push-button (PB) switch is closed, 12V from C3 is applied at A, causing T2 to turn on. This state grounds the Reset bus, turns on PFET T4 by putting V gs(on) on its gate through the ...