The app allows you to open hidden settings of the official VK client (which must be installed). You will be able to enable invisible mode, disable ads, and cache messages. It is not recommended to change other settings.
The invisible mode works for the VK app only. If you log in through the website, you'll become online.
Enabling the invisible mode does not change your status to "offline" immediately. If you were online, your status will remain the same for up to 15 minutes.