It's like the normal version, except very fast!
In the case of Google Instant, results start to show up as soon as you start typing. So if you wanted to find a page about "Vermont," you would see:
v... [results about "v for vendetta," let's say]
ver... [veryfine juice beverages]
vermo... [vermontster ice cream]
vermont... [normal vermont ads].
Kind of like that!