<p>Do they set <code>display: none</code> or <code>visibility: hidden</code>?</p>

<p>I'm pretty sure <code>display: none</code> takes the element out of the normal flow, whilst <code>visibility: hidden</code> just hides the element but still has a reserved space for it.</p>

<p>Should I just go download the unpacked version and study it or does someone have a quick answer?</p>

It uses
```
display
```
.
Edit: And from John Sheehan's comment on this answer, which I agree with:
You should go view the unpacked source anyway just to familiarize yourself with it
Edit 2: Comments have mentioned using Firebug to discover what is happening. Indeed, I actually went to the jQuery docs and used the Safari Web Inspector to see what happened on the
```
hide()
```
demo to be sure of my answer.