Alpha mask: show through any layer

An alpha mask shows a layer only where another layer paints, at that layer's transparency. Here a video shows through Spectrum bars.

Try this

  1. Press play with sound on. The video shows through the bars as they move with the song.
  2. Click the video and open Modifiers. Mask's Type is Alpha, and Layer names the bars.
  3. Set Type to Vector. The whole box opens, because a vector mask cuts with the outline.
  4. Open the bars' Spectrum settings and change Bars or Gap to reshape the windows.