The app coming home: the donor firmware was written for this exact board, and this is it returning through the convention, repainted 28 rows at a time on a device with no framebuffer.
This artifact is the full pack firmware for this board with fluidbox in its one app slot, not an app-only build.
No install needed: the download is one merged image (bootloader, partition table and app) written at offset 0, the same bytes this button writes.
Only needed if the board's own USB port never appears in the browser's list, which means firmware too broken to answer. Otherwise the flasher puts the chip in download mode itself.
Unplug USB. Hold BOOT while plugging the cable back in, then release it. The board comes up in the ROM download mode, its port appears, and this button works from there.