Thanks HN!!!
There are PCI slot brackets with power and reset buttons.
https://www.highspeedpc.com/ATX_Power_Bracket_p/atxbracket.h...
> Also great for isolating problem case switches!
... Which implies that that's not the primary use they envisioned.
Their main product appears to be an open-air ATX "case", which basically amounts to a motherboard mount, a fan mount, a rail to support PCI cards, and a mount point or two for hard drives, without side walls and without a power button. So they also sell a power button mounted onto a PCI bracket, to slot into an open slot on the case's PCI rail.
i.e. after press it returns to its previous state (momentary), and on press it 'makes' (as opposed to 'breaks') the circuit, or 'bridge' between pins as you said.
Which is why my screwdriver works - I momentarily hold it across the pins, making a connection between them.
But, y'know, it's not hard enough to pick up the screwdriver and poke (I do keep it right there, solely for that purpose) that I've bothered.