A bouncing square: the pack's own reference app, proving its band-render contract with nothing borrowed from another device.
This artifact is the pack's reference app, built into its one app slot: the same firmware, with the bouncing square as the app it boots.
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.