It's hard to breath. You let go of your paddle. The dynamics of the vertical pin with entrapment can be easily simulated by leaning a kayak against a grassy vertical slope of 45 degrees or steeper. When we first tried this exercise as part of a river rescue clinic, we were astonished by this discovery. According to conventional wisdom, a kayak must be slightly folded in the bow, snaring the paddler's legs, in order for entrapment to occur.
Gravity alone can prevent many paddlers from escaping a vertically pinned kayak.
Cockpit size makes all the difference. On the other hand, everyone could escape from boats with keyhole cockpits, such as the Prijon boats and the Perception Corsicas. In these boats both legs could be bent, then withdrawn simultaneously. Both feet could then be placed on the cockpit rim, allowing the paddler to dive or climb from the boat. In the Prijon boats, the forward end of the cockpit is slightly more narrow, initially allowing only one foot to be extracted to the cockpit rim.