Preview is a triggered ability. It works (theoretically) much like the Demon's Horn cycle of artifacts. When you cast a spell with preview, the spell goes on the stack and then the preview ability goes on the stack. It is important to note that the preview ability goes on the stack after the actual spell and will thus resolve first. It's also important to note that preview doesn't care about whether the spell resolves or not. Even if the spell is countered, preview shit still goes down (unless your opponent Stifles it).
Preview is like Threshold, Channel, Sweep, Radiance, Hellbent, Grandeur, Kinship, Chroma, Domain, and (yes) Gotcha - a pseudo keyword. None of these actually means anything by itself; all of them are simply ways of grouping a similar complex ability that appears on multiple cards. All pseudo keywords are in italics and are followed by a dash. They are used for abilities that do similar things (or have similar ways of doing things) but do not have an identical effect on cards.
Preview (and most of the others listed above) could have been made a standard keyword. Here's how it would have looked:
Preview - Target player sacrifices a creature (when you cast this spell, target player sacrifices a creature.)That's fine, I suppose, but it's kinda clunky. Less elegant, if you would. There are times when repeating yourself word for word is a good thing, but this is not one of them.
Oh, and please don't ask me how to pronounce Strzyga. I don't know and would be pretty damn embarrassed if somebody called me on that. That was the name of the art. I liked it so I lifted it.
Preview's kinda tricky to explain flavor-wise. The omen route worked pretty well for this guy (at least as far as I'm concerned), but Shardcrown Brute was just a complete bust. What are those ghosts doing there?