Thumb-Over and Arpeggios

I've been reading Chang's book and trying to implement some of the ideas. It has helped a lot so far, but I'm having trouble with arpeggios, especially with its description of thumb-over and thrust/pull. I wanted to see if anyone here has tried these and could provide some insight.

When ascending with the left hand, or descending with the right, is it correct that the thumb is not supposed to bend at all? Is it supposed to go under the hand at all, or is it more like a jump to the next note?

I'm fine with ascending in the right hand because I can just rotate and put my hand at the right position to put my thumb on the note without folding it under. But descending is the part that's getting me. Also descending for the left hand, for some reason, especially when the note right before the "conjunction" or transition to the next octave is far away from the next note. For instance I have a descending G minor arpeggio in the second inversion in the left hand in one section, and I find that difficult when going to the D. My thumb naturally wants to bend under to try to get the next note. On the other hand, I have an ascending G minor arpeggio in the right hand in one part and it is rather easy.

So if someone could clarify this, it'd be much appreciated.