Nit: that's not what negative reinforcement means. Negative reinforcement is about removing a negative stimulus, like inducing someone to go to a desirable website by improving their initially bad text contrast whenever they go there.
In this case, jumpscaring yourself would just be considered punishment (or "positive punishment").
* Positive reinforcement: [Adding] something so that entity does the action [more]
* Negative reinforcement: [Removing] something so that entity does the action [more]
* Positive punishment: [Adding] something so that entity does the action [less]
* Negative punishment: [Removing] something so that entity does the action [less]
P.S.: Note that this intentionally avoids diving into exactly how the Entity judges the Something. It's not always clear, even if in many cases you can guess.
P.S.: Sharing a book-quote that seems apropos, particularly the final two lines.
> People came, and tormented a nameless thing without boundaries, and went away again. He met them variously. His emerging aspects became personas, and eventually, he named them, as well as he could identify them. There was Gorge, and Grunt, and Howl, and another, quiet one that lurked on the fringes, waiting.
> [...] Howl handled the rest. He began to suspect Howl had been obscurely responsible for delivering them all to [the torturer] in the first place. Finally, he'd come to a place where he could be punished enough. Never give aversion therapy to a masochist. The results are unpredictable.
In this case, jumpscaring yourself would just be considered punishment (or "positive punishment").