So here is a little example of what not to do. Before I go any further, let just say that Progressive Disclosure itself is not bad. In fact it is a very useful tool in many situations where you need so show more options as a result of some previous option being selected. It is just this use of it that doesn't work.
I am in the market for a minivan so I head over to the Toyota site to see what it would cost for the one I am looking for. The site is looks very nice, however I ran into this little problem. The dropdown shown below has a clearly labeled link called "Build Your Sienna" as shown below.
When clicking it, the problem is obvious
I actually had expected to be taken to the "Build Your Sienna" screen rather than have this mysterious little form pop up. In fact, for a second or two I didn't even notice it. When I did notice it, I wasn't sure what I was supposed to do. So I just clicked "Build Your Sienna" again. While my mouse button was held down, the words "Zip code" were displayed in the form field. When I released the button, it went away again.
Since I didn't think my zip code was relevant, I just clicked on "Go" and was met with this:
Apparently, a zip code is required in order for me to build my vehicle.
First of all, the if the zip code field was going to be there it should be labeled initially. This could just be a bug due to me using Firefox. Perhaps the site wasn't tested enough.
In my opinion though, if a zip code is a required part of building my vehicle then it should be part of the vehicle building process rather than this unlabeled popup. The reason for the zip code is probably due to changing costs based on geographical location. They may not want to say that on the site, but they could have said something like "Your zip code is used to find dealers near you that might have the vehicle you build in stock" This would do two things, give the user a reason why the zip code is needed and also benefit the customer since there is a good chance that a potential buyer might want to actually go test drive the vehicle at a local dealership.
While it might seem cool to add the little form using Progressive Disclosure, this is not an appropriate place for it. "How" we put stuff on screen is only part of the equation. The "Why" and "What" are usually far more important.