According to the designer ”The Traccia is a phone concept modeled on how we use a writing instrument, specifically a pen. The intuitive way we write on a sheet is translated to commands for the phone thanks to the embedded optical sensor located at the end of the stick. Basically whatever you write gets translated to a command or function via the installed software. This means your invisible writing can be converted to SMS or an email, just like that”.
Source : yankodesign