Foot Entrapment when rafting?

Foot entrapment usually happens when people try to stand in fast whitewater after being dumped out of the raft or boat. Rocks in fast moving streams tend to be rounded and loose, often shifting around easily due to slime and wet moss growing on them that makes them slippery. Your foot can get wedged between shifting rocks and trapped. If there is pressure on the foot and shoe, you will not be able to pull the foot out. Since the ankle is the narrow part of the lower leg, it is most often the case that a shifting rock will press against it above the heel, making a more difficult entrapment than if it was just the lower foot If the water is running with a lot of force, this can make it even more difficult to change position or put enough leverage on the leg and foot, even with help, to wrench it free. And depending on the direction of the current, the water may pull you under as you weaken and are unable to keep your upper body above water.

I don't know why you have trouble visualizing this since you do describe the scenario. It's not a common occurrence, true, but it is a dangerous one that can be avoided by following the river guide's instructions to keep your feet up when dumped into the river and not attempting to stand.

Personally, I have had this happen to myself, though not while rafting. I was crossing a shallow river with a broken shale and boulder bed while on a long hike and a chunk of shale under my feet slid sideways and flipped 90 degrees, sliding my foot and hiking boot into a slot between two boulders and jamming it there. It made me fall over sideway, getting pretty bruised in the process. Fortunately the water was about up to my knees and not flowing too hard, so I was able to wiggle my foot and push on the chunk of slate until I had moved it far enough to pull my leg out. It would have been pretty scary if the water had been waist deep, though.

Foot Entrapment