The first version of hackerbadge just sent the private keys to the PoS device, and blind signed the transaction that sent the change back to the same address.
I loved the UX tho!
https://materialbitcoin.com/en/
It's funny because when, 8 or 9 years ago I saw them using cards with private keys in paralelni polis, I laughed at it.
Seriously though, wtf is wrong with people?
Discussion
The dark side of good UX can be that you mislead people who are most attracted to it. A good example is the bullshit on that page about "hacker proof", "robotic process" and "isolated network facility".
Just listening rn to a podcast with a guy who lost 1.4btc, using these (although I have no idea if these cards were at fault, the fact that he was even using them is already tragic).