A group of rebel fighters in Syria preparing to change their position learn what the fatal funnel is. Unfortunately for them, they learn through experience.

In combat, there's two ways to learn about something. There's learning through training and productive realistic rehearsals, and then there's the hard way. The hard way is when you learn something through injury or the loss of life. It's learning by experience. Unfortunately, when you're a rebel fighter or insurgent, military training is pretty hard to come by, and what we're seeing here is a group of fighters taking option two to learn a key lesson in urban combat.

The lesson here is that hallways and doorways are what's known as the Fatal Funnel. Anything that channelizes you into moving in one direction without giving you any real room to maneuver is a fatal funnel, and these things are best avoided at all costs. Unfortunately, urban environments are made up of an entire network of fatal funnels. Doorways, hallways, alleys, streets, all of these things are technically funnels that channelize you into one direction of movement and often don't leave you with any other positions of cover to maneuver to.

There's only one way to deal with a fatal funnel, and it looks a lot like eating a turd sandwich. You have to take one big bite and move through it as fast as you can hoping that you don't get stuck in the middle while some lucky machine gunner gives you the whole belt.