Look closely at a receptacle installed in the last fifteen years or so and there are small plastic shutters visible behind the slots. Push a single object into one slot and nothing moves. Push two blades in together, evenly, and the shutters retract and the plug seats normally.
That mechanism is the whole design. Children were being injured by pushing hairpins, keys and paper clips into receptacles, and the injuries were serious because a hand at floor level is often resting on something conductive at the same time. Emergency departments were seeing thousands of these cases a year in the United States before the requirement was written into the code for dwelling units.
The mechanism is purely mechanical, which is why it works without any electronics and lasts the life of the device. Both shutters must be pressed simultaneously to open, and the plug blades of any standard cord do exactly that. Anything inserted into one slot alone finds a blocked path.
There are practical consequences worth knowing. A new tamper-resistant receptacle feels stiffer than an old one, and older cords with slightly bent or worn blades can be genuinely difficult to insert. Pushing at an angle makes it worse, because the shutters need even pressure. The correct technique is to align the plug square to the face and push straight in.
They are not a substitute for anything else. A tamper-resistant device does not detect faults, does not protect against shock from a damaged appliance and does nothing about an overloaded circuit. It solves one specific problem, which is objects being poked into an energised opening.
When older receptacles get replaced during any electrical work, the new ones will be tamper-resistant by default. That is worth expecting rather than being surprised by, particularly in a house where cords have been in service for decades and the blades no longer sit perfectly flat.
There are a few exempted locations where the requirement does not apply, generally receptacles well above floor level, those dedicated to a single appliance that is not easily moved, and some in non-dwelling settings. Those exceptions are narrow, and installing the tamper-resistant version anyway costs almost nothing and removes the question.
Background reading: originally posted on Coe Schule