Developers are building iPhone Duo apps that use the hinge as a control, not just a way to get a bigger screen into a pocket.
The clearest examples are small, single-purpose paid apps. Toastie ($2.99) has you assemble a grilled cheese sandwich, then close the Duo to toast it. Fold Accordion ($2.99) uses the hinge as the bellows. Deck-Man ($4.99) has you pick music on the open display, then snap the Duo shut to start playback. Duometry ($2.99) asks you to fold to a target angle and lock in your guess. Orisuji ($4.99) is a paper maze that moves from the inner display to the outer one. Fold & Squish is free: you paint on one half, close the device, rub the outer display to smear the paint, then open it to see the result.
Some apps borrow old hardware instead. Tiny Computers ($3.99) mimics machines like the Macintosh, Commodore 64, Amiga 500 and iMac G3, with clickable keyboards and hidden easter eggs. The free Delta emulator now gives the Duo Nintendo DS controls, and closing it looks like the DS shell and puts the game to sleep.
Not every app depends on the fold. Living Earth ($5.99), a weather and globe app, is aimed at tent mode. Portal, a relaxation app with over 100 locations, costs $2.99 per month or $29.99 per year. Lake, a coloring book, wants $9.99 per month for the full version.
The pattern echoes early iPhone novelty apps, where a new sensor or gesture spawned a wave of gimmicks. Most hinge games are cheap one-time purchases at $2.99 to $4.99, which suits an idea that may not hold attention for long. Delta and the TypeDraw notebook ($4.99, Apple Pencil support) look likelier to last, because the fold improves a task people already do.
The odd one is Fold Focus, a pomodoro timer that rewards you for keeping the Duo closed while you build a virtual city. It costs $5.99 per month, a recurring fee for an app that works by getting you to leave the phone alone.