When you see something you want, you can grab the unit right then and there -- you just hit exchange, pay the fee, and it will go into "pending confirmation" status. II holds the trade for roughly 14 days (don't quote me on that timing -- they send a postcard if Starwood doesn't confirm right away but I can't remember the exact timing of the "we're going to cancel on X date if your resort doesn't confirm" message). Then, the tricky part begins -- getting Starwood to deposit something of equivalent trading value. When I do grab a trade, it's with a week I've reserved --- it helps my argument (and, believe me, I've had to argue). I tell them they have to deposit the exact week I've reserved or something equivalent that will qualify for the same trade. I've had to get supervisors involved, but it's worked successfully. Although most of their employees are pretty clueless about II trading, you can get them to understand that you won't get the trade if they deposit a crummy week.
Important note: I don't wait for the postcard -- I start calling Starwood immediately after trading to begin the process of getting them to confirm the week. It's painful -- I had hopes it was getting better after Pat (grayfal) reported successfully depositing reserved weeks. Then, there were a few reports of Starwood denying deposits ... so I don't know what to expect this year as I move into "prime reservation season" for BP. It's extremely frustrating ... I own lots of weeks ... several different mini-systems, and Starwood is the ONLY one that makes the exchange process painful. Although I don't like it that they control deposits for SVN weeks, I understand that they have the right to do so -- it's written into the contract you sign. But -- they have no right to control deposits for non-SVN weeks -- they can't have have their cake (deny membership to SVN) and eat it too (try to enforce SVN's rules).