What are double-action key switches good for?

[Findecanor]

There are a few rare switches that are double action, meaning that they have one actuation distance and a second actuation if you press it deeper. I see that some have been found in typewriters on Return, Space and Backspace keys...
But there is something that I have missed...

What were double-action key switches used for?

And ... what would you use them for?

[Daniel Beardsmore]

The patent for [wiki]Olivetti snap action[/wiki] does note that the second stage can be used for key repeat (this form has not been seen, but it is illustrated in the patent).

I have one and a half double-action switches, and stage two is heavy enough to drive a family car over it. To get key repeat you'd have to put the key in question in a vice. The Alps specifications suggest something as heavy as my Mitsumi switch.

I don't think anyone's ever proven the function of the keys, which is curious as several people do have typewriters with these switches in (Mitsumi, Alps, Marquardt, but not Cherry yet so far as I know). Maybe they have — the wiki is only a fraction of the knowledge gained and lost within the forums.

[Phenix]

one can code somethink similar with QMK (do 1, 2, 3.. If tapped, do F1, F2, .. if held for X ms)

[Ratfink]

The most common use is probably pressing harder for key repeat. At the very least, that's how the Selectric spacebar works. Honestly, I kind of like that more than the software delay we have today.

[Findecanor]

There we got confirmation of key repeat. Thanks!

[ullr]

I think the most interesting use would be for simultaneous modifier keys. I’d like a layout with one under both Controls mapping to Shift, and two under the Alt keys mapping to Shift and Control respectively so I can have two alts, Alt Gr (via Control + Alt), and simple two‐key cords for all common key bindings in a very tight form factor. The Menu/Application key also seems to activate on upstroke in most contexts in my experience, so it’s ideal for having another key “under it”, perhaps a macro key. I think a dual FN/FN2 key would be perfect for 40% layouts, as well.

[Chyros]

IIRC they are now using double action switches to develop analog-like keyboards for gaming.