The Retro Vector combines two techniques: G-Stop and Triangling. A G-Stop is when you let go of the gas button while you're hooked on to something and passing it, slowing you down to a stop. For this technique, the traingling is delayed and utilized after the G-Stop. It is useful in situations where you find yourself approaching a titan at an odd angle or altitude and are too far away to perform an airquator but too close to perform a vector cut. While it does not conserve your momentum well and is therefore not an ideal technique for killing titans, it can come in handy when you get your spacing messed up or find yourself in a situation that is difficult to recover from otherwise.
This technique needs to be used at high speeds to see success. For smaller titans you can go a little slower, but taller ones require more velocity. This is because the Retro Vector relies on the delay period after you pass a titan before it turns around to face you. That being said, I have only tested this technique on Normal mode and have no idea if it works on Hard mode, where titans react faster to your actions.