The Wing's second screen can either act independently of the larger screen while it is displaying a film, for example, or can complement it as a controller for gaming and media.
However, some of this functionality relies on using pre-installed browsers.
One expert said it was one of the "more radical designs" he had seen.
"It's a very tricky design to evaluate without using it - it is reminiscent of some of the designs we saw from Japan back in the late 2000s," Ben Wood from the consultancy CCS Insight said, noting it was unlike any of the folding-screen designs that several of LG's rivals have recently focused on.
"You have to applaud LG for experimenting. Ultimately it will be up to consumers to decide whether its an approach that works."
The 5G device also has three cameras on the back and a fourth pop-up lens.
LG said the Wing phone would initially launch in South Korea next month.
North America and European launches would follow, it added, but the handset price has yet to be revealed.
The firm has taken some steps to reduce potential damage to the amount of moving hardware on the device.
The internal accelerometer, which monitors motion, detects if the smartphone is being dropped and the pop-up camera will retract if it is in use, LG says.
The hinge of the swivel screen is equipped with a hydraulic shock absorber, and the back of the main screen has a coating to prevent it scratching the smaller screen underneath.
LG claims the hinge is "still reliable" after 200,000 swivels, although it's not clear how much time this would be likely to take.