Task initiation requires a massive spike of dopamine to overcome the resting 'friction' of the brain. In ADHD, baseline dopamine is chronically low. When confronted with an undefined, multi-step task like "write an essay," the prefrontal cortex attempts to simulate the effort required. Because the task is undefined, the simulated effort appears infinite.
When the brain calculates that the required effort vastly exceeds the available dopamine, the amygdala (the emotion/threat center) hijacks the circuit. It interprets the massive cognitive load as a physical threat and triggers the "freeze" state of the fight-or-flight response. Your physical paralysis at the desk is an evolutionary survival reflex to an overwhelming cognitive demand.
Furthermore, the "Blank Page Syndrome" specifically attacks the impaired ADHD working memory. To write an introductory sentence, you must hold the entire thesis of the paper in your mind while simultaneously managing grammar, tone, and typing. This cognitive overload causes the working memory buffer to overflow and crash, leaving your mind completely blank.
