While wandering the halls of Mobile World Congress, we came across NUIA (Natural User Inter Action), a German company whose software is designed to make it easier for developers to code apps that make use of eye tracking sensors, such as Tobii's.
If someone tries to contact the user, their action is recorded by Microsoft's servers, which then send a notification, reactivating Skype so it can handle the activity.
For a plain-English explanation of how GRATs and other estate planning opportunities work, see Estate Planning Smarts: A Practical, User-Friendly Action-Oriented Guide, the bestselling book by Deborah L. Jacobs .