The wire in this “engine” has what is called “shape memory.”  When heated beyond a certain temperature, the metal shrinks to its original “remembered” dimensions.  Otherwise it may be stretched with little resistance.  So the pulley nearest the hot water bath has a greater mechanical advantage and will result in wire drawn into the hot bath.  With ice cold water on one side and boiling hot water on the other, the engine will spin quite rapidly (100 rpm) after given a starting spin.

Nitanol Heat Engine