1) Install OL module and geofield
2) Clone the "Geofield widget map"
3) Check the "restrict extend" checkbox.
3) Set the center of your map. Shift-click and drag to set the extent.
When I try to set the extent, the map itself behaves as expected, I get a white semi-transparent box with a red border over the restricted area while dragging. When I release the mouse however, I expect the settings to get copied over to the text box below the "restrict extend" checkbox, and possibly the extent to remain visible on the map. Neither of these happen however, which looks like a bug to me.
I'm not very good with JS, but I looked in the Firebug console to see if there was an error message that might provide a clue as to what was going on, but sadly that gave nothing.