A Green Circle (Go) or Red Triangle (No-Go) will flash briefly. Press Space or the button only when you see the Green Circle. Do nothing when you see the Red Triangle.
Respond quickly to Go stimuli. Withhold your response on No-Go stimuli.
Go/No-Go separates automatic responding from stopping an already-started action. Commission errors (pressing when you should not) are a straightforward read on impulsivity and inhibitory control for that session.