From publisher blurb:
What is a GUMSHOE Zoom?
Not everything can support a game of its own, or even a big sourcebook. For those things, we present the GUMSHOE Zoom, a sort of supplement focused on a key game mechanic and its possible applications. In general, Zooms are interesting potential hacks, or intriguing adaptations of the main rules. Some apply to one specific topic or sub-sub-genre. Others cross all manner of GUMSHOE turf; you can slot them in and adapt them to tales of Cthulhuoid investigation, mean superpowered streets, or alien colonies alike.
Zooms are experimental. That does mean that they haven’t been playtested, necessarily. (If something in here is really really broken – and it’s not, as this ain’t our first rodeo – we’ll fix it in post.) But that also means we encourage you to experiment with them. Changing the cost, or prerequisites, or point effect, or other mechanical parameters of a given Zoom changes how often it shows up and how much drama it drives. The dials are in your hands.
Zooms will change the focus of your play if you use them. Putting a mechanic on the table puts it into your game. Adding a Zoom means more actions, possibly even more scenes, using those rules. Since the Zoom mechanics are intended to encourage specific actions or flavors, to force a card in your storytelling hand, they aren’t “balanced” against “normal” actions or rules. In general, if you don’t want to see more of it, don’t Zoom in on it.
Zooms are optional rules. You can and should ignore them if you don’t want them, or change them at will. After all, if a given Zoom turns out to be crucial to an upcoming GUMSHOE game, we’ll change it to fit that specific genre or form of storytelling.