On iOS the input that triggers the "undo" feature is shaking the phone.
It's so fucking stupid I can't believe anyone came up with it and people took a look at it like LGTM, and it went through implementation and testing and no one ever said "this is fucking bullshit"
The devil, right after inventing tap-to-click: "Wait, I got another idea."
I recently realized you can trigger this by shaking side-to-side like waving, before I was headbanging it.