A snag line is a great way to dislodge a trapped foot. Often, you will need to weight down the line just downstream from the trapped foot. From this point, walk each end back upstream hooking the shin / ankle and freeing the extremity. Make certain you have some way to retrieve the victim quickly since the probability of injury is pretty high and more than likely they will have swimming difficulties.
Take a closer look at the photo. This is a good example of a weighted snag line rescue. The first step is stabilizing the victim by running a line across the river and providing him some support. The stabilization line crosses him roughly chest high. The next step is weighting a line in the middle and dragging back upstream so you can take loosen the victim's foot around his ankles.
A far better method is sawing the snag line downward. Rescuers wade out into deeper water on both sides and simply pull the snag line back and forth in a sawing motion with downward pressure. Once the line is low enough, typically around the ankles - both sides can move upstream to make a deep V and pull the leg/s free.