En este tutorial se explica cómo esnifar una red o un equipo en concreto sin que nos coloquemos fisicamente entre él y su puerta de enlace. Un escenario ideal es hackear una red wireless, nos conectamos a la red como si fueramos un host más, y usamos esta técnica para que todo el tráfico pase por nosotros, y lo analizamos con el software que sea, o mismamente con el Ettercap. Este sería un ejemplo lógico de qué es Man In The Middle:

Sniffing techniques are becoming more popular every day, Arp-Spoof, dns-spoof, man in the middle, etc, But there are many who believe that they are reserved for the gurus of the network, and until recently it was like that, but in this as in everything, over time, The Internet has made it possible to rapidly develop tools that make what was previously very complex, Now it's a child's thing (I'm not sure if that's good or bad).