Why the file never leaves your browser
Almost every background remover uploads your photo to a server, runs the model there and sends an image back. That is simpler to build and it means your picture — often of a person, often a passport or ID photo — sits on someone else's infrastructure under a privacy policy you did not read.
This runs the segmentation model in the browser instead. The model is downloaded once, a few megabytes, and cached. That download is the only network traffic the tool produces; the photograph is read from your disk into memory and stays there.
The trade-off is honest: an in-browser model is smaller than what a server can run, so a complicated edge — flyaway hair, a fence behind a subject — will be less precise than a paid cloud service. For the common cases, a person against a room, it is very close, and the feather control hides most of the difference.