Final answer:
The Gestalt principle at work during a ventriloquist act is the law of continuity, which makes us perceive the dummy as the source of the voice.
Step-by-step explanation:
The Gestalt principle that is at work when a ventriloquist moves the dummy's mouth while doing the talking, making it seem like the dummy is talking, is known as the law of continuity (or good continuation). This principle suggests that we are more likely to perceive smooth, continuous forms rather than disjointed, broken lines. Our brains connect the ventriloquist's voice with the movement of the dummy's mouth, leading us to perceive a continuous action of speaking as coming from the dummy. This illusion takes advantage of our natural tendency to group elements together into a coherent, flowing form.
This creates the illusion that the dummy is talking.