The vent opener spring-arm mechanism has a hydraulic cylinder threaded into it that is filled with a mineral wax. When the wax is heated, it expands and extends the piston out the top of the cylinder. When the piston has been sufficiently extended to overcome the spring force and window weight, the arm begins to extend, opening the window. When the cylinder wax cools, the piston retracts with the aid of the springs and the window gradually closes again.
This link has the most detailed explanation of this automatic vent opener mechanism that I've found (under the 'More Product Info' tab):
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