Thx for the answer.
I have the Sandor too
The thing I have found is that for arpeggios for example C-E-G..., when you pass the thumb, you really pass over (in the front of
, in fact) the 3rd finger, fingers are never really one behind the other at the same time, but the thumb lands on the keyboard on the same place that if there was a real crossing in the front of, this allow you to keep the same angle with the wrist during argeggios (if you pass under, you have to move your wrist to the left and to the right)... The 3rd finger bent over and continue his motion "towards the palm", this last point is hyper important. Generally, you push the finger and raise it in the opposite direction, the motion is discontinuous. But if you pursue the motion towards the palm, it makes a circle with your finger. This give you space for the thumb to pass in front of.
For arpeggios, it doubles up your speed immediately in comparison to the thumb under (try the Op10/8 from Chopin etude).
Thus the key is for me make this circle motion, and playing the thumb with a diagonal motion (to the right here and towards the hood) + a rotation.
During this thumb motion you have the time to reposition the circled finger by ending the circle.
For scales, you can do a thumb "in front of" for the 4-1 passage, but for the 3-1, this is doable but hard.
Thus I considere scales like a particular case of argeggios, an hard case.
It gives the sensation of never passing under the palm because when you pass in front of with the thumb, others fingers pass in front of too. And yes, there is probably almost no lateral motion of the thumb.