As I interpret it, it's the former.  If you deactivate exclusivity, it will take effect 30 days later.  If you change your mind, you have to wait 90 days after exclusivity took effect before you can reapply.  For example, if you cancel your exclusivity on April 1, 2013, it will take effect on May 1st, 30 days later.  If you want to become exclusive again, you can't do that before July 30th, 90 days after May 1st.

Also note that if you change your mind while the 30 day clock is running, there is no going back - so you can't just cancel your cancellation as it were.

I asked support about this in early 2011 when i was trying to decide if I'd leave exclusivity. That is a pretty big financial penalty if you start the 30 day clock when you aren't sure. You'd not want to upload anywhere else for just 90+ days, so that'd mean that you'd wait out the 90 days with indie income from iStock and no income from other sites to make up the difference.