DIFF is a spot-the-difference game: two versions of the same image side by side, a handful of small changes between them, and a timer. The differences range from obvious to genuinely cruel — a shadow's angle, a missing button.
It's a pure observation test, and the reason it's satisfying is the same reason hidden-object games are: the moment your eye finally resolves something it has been passing over for a minute.
Usually yes — a miss costs time, which discourages clicking randomly at the end of a stubborn level.
Scan the pair in systematic horizontal bands rather than looking around freely, and check shadows and colours as well as whole objects.
Yes — tap the differences. Nothing to install.