It took me freaking 1 year or longer (since I do it) to figure out how this works. And finally I know it. It works on every guitar by the way!
Also, it is super consistent with the thumb if you simply keep in contact with the strumbar the whole time. When you have your thumb bended like this, it will automatically do this double upstrum.
If you use other fingers or so, the chances of slipping off are enormous and it would be way more complex.

This always was a mystery to me since even though every kind of overstrum (no matter if slip-back or double down/upstrum) is fixed, it still worked. It basically is a completely legit strum if you see it this way.
Here the explanation for you!

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