I think the best way to really understand the essential up/down motion of the TH is to do a single Barrel Roll motion from the Split Bottom mount and immediately follow up with the reverse motion.
Split Bottom → Underpass → Reverse Underpass back to Split Bottom
The key here is to perform the underpass as a “lift-over” instead really passing under with your NTH index. If you work on this back and forth underpass in quick succession you will work out the up and down motion to perform boings.
Once you are comfortable with quick underpass-reverse-underpass motions, try to do an underpass and reverse-underpass in one motion without the yoyo stopping. That is basically your first boingy-element. From there it’s straight forward, do more and more back-and-forths in one step.
Here’s a video showing the concept: