The keyword "Holo Flash" implies two things:

: One of the most popular apps for this purpose. It provides a gallery of 3D videos (hummingbirds, planets, etc.) designed for pyramid projectors. Hologram Maker

If you want to project your phone screen onto a wall for free, you can build a :

Some advanced apps sync the phone’s LED flash (torch) or screen brightness bursts with the hologram movement. That’s the "Flash" in Holo Flash.