A word will appear. Write your response sentence before the next word replaces it — that's the entire discipline of the real test. Responses stay in your browser only; nothing is sent or stored anywhere. You can pause for a genuine interruption, but every pause instantly costs 3 seconds off that word's remaining time — the real Board doesn't stop for you, so use it sparingly.
Real practice value comes from doing this daily, not from one long session — a short 15-word drill every morning builds the reflex faster than occasional marathons.
This is the exact discipline Chapter 6 of Chest No. 80 covers in full.