Generally speaking, smart windows rely on electrochemical reactions in order to change transparency. This can make for an expensive manufacturing process, along with the use of sometimes-toxic substances.
The Harvard technology is different. It incorporates a sheet of regular glass or plastic, sandwiched between two clear elastomer layers that have been sprayed with silver nanowires. Those wires are small enough that they don't affect light transmission on their own.
However, when an electrical current is applied (simply by flicking a switch), the nanowires on either side of the glass are drawn towards one another. This causes them to compress the elastomer layers, distorting them in the process. As a result, the window goes from being completely clear to taking on a cloudy frosted-glass appearance.