HANGING!

Hanging: attacked and guarded by nothing.

Under-defended: guarded, but if the other side starts trading on that square, they still come out ahead. A cheap attacker on an expensive piece counts. Too many attackers counts. Pinned pieces don't count as attackers.

Tap every piece like that, white and black. Kings never count. If you think nothing is loose, just hit Done.

After a board, tap a note to flash its piece on the board.

Tap the bookmark at the top while playing to save a board. Saved boards are under Saved Games, with a link to look at them on Lichess.