Hikaru Shiina keeps the lingerie on

Hikaru Shiina works through her lingerie rather than out of it, pushing the fabric aside with one hand while the other stays busy. Her legs stay open to the lens, knees drawn up, and the angle holds low and steady from start to finish.

Amateur Japanese films like this live or die on how little they interrupt. Here the camera stays put, the room light is flat and ordinary, and her breathing carries the scene. Masturbation filmed this plainly reads as private rather than put on for show, which is why it holds.