The photo is a bit rubbish, but looks as though it is fitted similarly to draw reins (when used for lunging obv!) - Just stops the horse sticking its head up.
This is my Favorite type of lunge aid I have always called it a lunging rope mines not quite the same as it adjusts with a toggle in the middle and in made of smooth rounded rope .
It passes over the back though the forelegs to the bit has the great advantage of not overbending horses not fixing them it excellent for working long and low with young and weak horses they also can't lean on it at all.