I've only seen these in action by accident. Happened to me when I tried to go around a planter to reach the eating area at a Whole Foods. Apparently that was beyond the perimeter and I got stuck.

I saw one just last week lock up right at the door of Safeway when a lady was trying to go out to her car (it's not supposed to block you from exiting the store). She was stuck in the door, cart half outside.

But people who actually do want to take the carts away know exactly how to avoid the mechanism.