I was in a workshop with Alexis and Wen-Hsiung, the compact explanation they gave is: in oder to avoid turning, the right hand should go up when you are pulling the diabolo towards you.
Now, let's expand:
- Accelerating in vertax consist of pushing the diabolo away form you (frontwards), and pulling the diabolo towards you (backwards)
- If right hand is lower all the time, one needs to turn
- If right hand is lower only when pushing and up when pulling -> good, you need not to turn
- If right hand is higher all the time, you will go out from vertax and eventually do a front-flip, not recommended. Just kidding, there are other vertax positions that hold this, Marko explored them pretty well, but this film is not necessary for your question, just informative
For your question: the key is the motion to switch the height of the right hand in between pushing and pulling, right hand should be higher when pulling.
I hope that our explanations do not confuse you, and that mine is correct, I ain't no vertax expert.