The app
Video and a controller
A picture locked to the song, and knobs on a MIDI controller.
Video
Add a video picks a movie from this computer. It stays in the browser for this session. It is not uploaded with the song. While you play, the picture follows the playhead, including inside a loop. Closing the tab drops the picture.
A controller
Connect a controller asks the browser for MIDI. Chrome can allow it. This embedded preview often blocks it. Channel 1 is the first part that is not a spacer. The volume knob moves that fader. The pan knob moves it left and right. The center of the pan knob is the center of the song.
Several knob moves close together become one undo. The mix updates on screen immediately and saves a moment later.
Record a part
Record a part, in Pro, takes one input through the browser and adds it as a WAV stem. It is one take from one input. It is not a multitrack recorder, and it does not overdub on top of itself while you hear the other parts through a cue mix.
Around you, the video, and the controller are previews and helpers in the page. The file you save is still the stereo mix. See Play and compare.