The “tether” in this sense is the automated downloading of new images as they are taken.
When the tether is started then any new images created on the camera, be it by pressing the camera shutter release or by this script, will be downloaded at once and will be listed and displayed in the preview window (if jpg).
If tether is stopped then files are only saved on the camera card.
If tether is started the files are saved on the camera card and downloaded to the PC. This does mean some delay of course, which depends on the size of your image files.
There is no way to have the image downloaded without saving it to the camera card.