Okay, so you're over in the media forum talking about how Darth Vader is Luke's.....
[hidden]FATHER!!!!!!!!!!!!!!!!!!!!!!!!!!!!![/hidden]
......but you don't want to spoil it for everyone who hasn't seen the movie yet. Or, maybe you're playing through a Zelda game and you've discovered that.........
[hidden title=Who could it be???]GANNON!!!!!!!!!!!!!!!!!!!!!!!!!!!!!![/hidden]
.....is the final boss. We don't want to spoil these kinds of things for our fellow board members, and rather than just typing out an easily-missed "spoiler alert," you can use these nice spoiler boxes to stop the....
[hidden style=2]SPOILING!!!!!!!!!!!!!!!!!![/hidden]
Okay, so maybe I got carried away on that one. At any rate, to use these tags, just type them out as normal YaBB code:
{hidden}blablabla{/hidden}
As always, remember to use square brackets instead of the squigly ones in the above example. If you prefer to use a spoiler box like in the third example above, you can set the parameters like so:
{hidden style=2}blabla{/type}
Titles for either type of spoiler box can be used as well. However, the title (in the form title=xxx) must come after any "style" setting. If you are going to use multiple spoiler tags in the same thread, they must differ slightly, like so:
{hidden}blabla{/hidden}
{hidden style=1}more blabla{/hidden}
Both of the examples there will look identical in the message view (since style 1 is the default), but YaBB will run the code together unless they are slightly different. If you're confused by that, just do one hidden tag per post and all will be fine.
Thanks go to Rick (shady tool) for suggesting this mod ages ago, my laziness for not finding it sooner, and the people at Board Mod that coded it.