Perception of predeterminism
Feelings of predeterminism can be defined as the sudden perspective or feeling that all events, including human actions, are established or decided in advance.
This is a perspective that can become spontaneously triggered and felt through a distinct change in thought processes and an intense set of physical sensations. In terms of how it feels, the perspective can be described as the assumption that ones internal narrative possesses "free will" and is therefore an independent decision-making agent becoming revealed as illusory.
This revelation occurs through a forced change in perspective. It creates the undeniable sensation that ones personal choices, physical actions, current situational perspective, and the very subject matter of their thought stream has always been completely predetermined by prior causes and are therefore outside of conscious control. Instead, ones decisions and actions are felt to have as a vast and complex set of internally stored, instantly decided, pre-programmed, and completely autonomous electro-chemical responses to sensory input.
This experience as a whole is consistently interpreted by anybody who undergoes it as a profound revelation or insight into the illusory nature of free will.