I don't think I can fully represent all 3.5 billion owners of testes, but I'll do my best.
Firstly testes only produce sperm, which is a small percentage of the total volume of ejaculant
IIRC 4 organs contribute to the fluid with the seminal vesicle making up about 3/4 of the total.
So there aren't great volumes of liquid sloshing about, like milk in a churn being turned into butter.
In writing it is mostly used to indicate virility, much like excessive cum or excessive viscosity of cum.
It may also be used to indicate response to stimuli if the subject is being uncommunicative [as gentlemen are want to be] or as a prelude to orgasm
You could consider it analogous to cervical tenting
This brings me on to the one sliver of truth that can be found in this. If rather than the contents of the testes churning it is viewed that they are moving around in the scrotum this does occur, if rarely.
During ejaculation the scrotum can experience a series of contractions in time with the muscle contraction that cause each spurt. Prior to this there can occasionally be a number of preparatory spasms, the status of the prostate preventing these becoming an ejaculation. But even if you are trying to keep the guy on the edge this will not last long and will have the level of coordination of a death spasm.
There is one more technique that may produce this, don't try this at home as you will get punched.
When the testes are particularly low hanging [ they like to be 2C cooler than the body so move away to cool off]
Tap the cremaster muscle which controls the distance from the body it will spasm and you can get the testes to do a little dance.
Having been on the receiving end I can confirm it is very annoying and will lead to an ache that lasts for hours.
Hopefully you will have found this the first useful piece of mansplaining you have received.