As a general rule, it is best to get a boat or person out to the entrapped victim. Chances are they are injured and hanging on for dear of life.  The constant force of water pounding on them will wear them down eventually - time is critical.  While safely getting someone to the victim, others can work on getting a rope across the river to work as a stabilization line.  Once the victim is stabilized and no longer in danger of drowning, work on freeing them.  You may need to manually free their foot which means putting yourself in danger as well.  Make certain you have safety in place such as a tag line on a rescue vest.  Safety will need to be set both downstream and upstream.  Take very careful scraping steps and use a paddle or rope as a third leg.  Once the victim is freed, they will certainly need help getting to shore as they are most like injured.

Entrapment can be avoided by staying away from strainers, sieves, undercuts, and proper swimming techniques if you come out of your boat.  Don't try to stand in fast current up to your thigh area, chances are you will get swept downstream out of control.  If you have an older boat, make certain the walls are very secure so it is unlikely to collapse.  When purchasing a boat, make certain it has a full bulk head for your feet instead of foot pegs that were common on older boats.  Make certain throw ropes are stored properly and carry a rescue knife in an easy to get at position on or in your PFD.  Just like skiing, stay in control.  If the run is too difficult, portage the tough rapids or walk out if necessary.  Here is a good article describing foot entrapments.

Foot entrapment rescues can be really dangerous.  A combination of a stabilization and snag lines may avoid a hands on rescue but this is very tricky to pull off, especially the weighted snag line.  Another alternative is the saw method.  Run a line at water level downstream of the victim.  Wade into the deeper water on both ends and pull back and forth pressing downwards.  This approach is much faster and has a higher chance for sucess than the weighted snag line.  Most of the time, someone needs to go out to the victim.  Be very careful with footing so you don't fall into the same trap.  Use a rescue vest and secure a line to your tether as an extra safety precaution.  Stir the bottom with your un-weighted foot to find the next safe place to plant your foot.  If it grabs it should be easy to pull back out.  Once you make it to the victim, you should be able to pull out an entrapped foot fairly easily.  To unpin a boat, have someone hold the victim's body to stabilize and try to back out the boat or loosen one end depending on the pin.

If the victim lacks a rescue vest and it is too difficult to pull them out with a snag line, try a cinch line rescue. The cinch line is an extra line above the snag line that attaches with a carabineer. The send line forms a loop that tightens around the victim enabling a more controlled pull. Consider tying a stopper knot in the middle of the line to limit how tight this loop gets. Here is an article demonstrating a cinch line foot entrapment rescue: Cinch Line Rescue.

NOTE: There are many variations of cinch methods you can choose from. A huge advantage of a cinch over a snag line is you maintain control of the victim. This can be really important since the victim probably has serious physical injuries and might not be able to swim.

SnagLineA 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.