There is a mechanism in the intake manifold that changes the length of the runners. If you look under the spark plug wires top front of the engine, you can see the actuator. It turns a cylinder that extends through the intake manifold, that has holes placed to change runner length when the cylinder is rotated.

Anybody know what conditions trigger the actuator?

I don't have a problem with it, I'm just curious. I would assume this switches over at higher RPM but really have no idea, and am wondering if anybody knows.