There are lots of ways to pin a boat (or body). Any hard obstruction that stops your boat can quickly turn into a pin. If you run sideways into a boulder, learn to lean into it so the river current can pass underneath your boat and sweep you around the boulder. If you lean back upstream, you will definitely flip, probably come out of your boat which will quickly fill with water and wallpaper the boat to the rock. The force of water going downstream is often very strong. Once the ends fill with water, the boat will probably need some assistance to free it. Dropping off a tall ledge or falls, the front of a boat may get trapped under a rock. The back of the boat is often held in place by a crease in the drop or and indention. Water and gravity from the falls helps to press the boat into a stable pin position. This is an example of a vertical pin. Novice boaters often get turned sideways and in shallow rapids both ends of their boat get hung up. There are other ways to pin a boat but these are the most common. In general, pins are caused by obstructions (an immovable force), strong water current and gravity to hold you against that obstruction, and often friction between the boat and rock that makes it hard to dislodge.