A Passthrough E-Cig mod is a cord connecting the electronic cigarette to an external power source such as a computer, portable power unit or a videogame console providing a continuous power source.  What you tend to see with most passthrough’s are USB cord connecting to the electronic cigarette or an inlet to plug in a USB cord to the electronic cigarette.  Some passthrough’s connect direct to the atomizer, which only allows you to use the electronic cigarette while it’s connected to the power source and others connect to a battery, which could then charge while connected to the power source and hold it’s charge when off.