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 an undeniable change in thought processes and an intense set of physical sensations. In terms of how it feels experimentally, the perspective can be described as the sensation of the ego or internal narrative (as an independent decision-making agent) being lifted from your perception of the world and revealed to be illusory. This creates the undeniable sensation that your personal choices, physical actions, current situational perspective, and the very subject matter of your thought stream have always been completely predetermined and out of your control. At this point it becomes clear that these things are not reached through a conscious decision making process or planning of the ego and genuinely never were at any point. Instead they are revealed to have always been a vast and complex set of internally stored, instantly decided, pre-programmed, and completely autonomous electro-chemical responses to received sensory stimuli which one does not have any conscious control over.
Alongside of this, there is also a powerful physical sensation that makes the precise arrangement of the external environment and the objects within it become felt to be the true deciding force in regards to the way in which you physically interact with it. This is done by instantly triggering your responses and decisions towards them through the simple perception of them. This can lead you to feel that, for example, you are not deciding to reach out and grab a relevant object, but the object is triggering you to reach out and grab it because you need to (regardless of whether or not you have put any thought into it).
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.