The ActivatableButtons and the X-O game samples make use of the so called Activatable Behavior (or EyeX Click). Nothing happens until the trigger key for the Activatable Behavior is pressed, which is the Applications Key:
Activating an activatable interactor
So, the idea with this behavior is that you look at the button (or region) you want to activate, and then press the applications key to actually click it.
This is an example of a multimodal interaction that cuts away the need for a mouse cursor altogether. Just point with your eyes, and click with a key press.
An advantage of using the Activatable Behavior is that it has a built in snapping behavior, so your eye-gaze only need to be close enough to activate a button, and that it does some other clever processing behind the scenes.
Included in the Activatable Behavior is also events that you can use to highlight a region when it has gaze focus. You can read more about the behavior in the Developer’s Guide.
Regarding the reliability of the EyeX software, I would say that the functionality we have written samples for are reliable to build on. We plan to expand the set of samples as the API matures and features are added.
For your application, maybe you could make use of the Gaze-Aware behavior, which generates events when the user’s eye-gaze enters and leaves a specific region on the screen? This behavior is also described in the Developer’s Guide. Depending on what you want to achieve one of the data streams may be suitable as well.
I hope that answers your questions and that the samples work as intended using the application key to trigger the EyeX Click.