The way it detects a leak varies but typically it uses a vacuum or air operated pump to pull/push a diaphragm pump. The other side of the diaphragm tries to pull vacuum on the evap system or pressurize it.
Once triggered it looks at the state of the pressure switch. If within a period of time the switch closesor not it means there is a leak and it then cycles a number of times again and measures the cycle time to determine the size of the leak. On a Jeep I think it mesures how long the pump runs to determine the size of the leak.
To me that code suggests that the switch is always closed. That could be a massive leak right in the area of the evap cannister or a bad switch. You can check the switch yourself with a simple ohmmeter. With the engine off, it should be open. If not you have your culprit.
Check with your respective service manual to verify this is how YOUR evap system works. As I said they vary, but all follow the same basic idea. For example your switch may be closed then under vacuum/pressure, opens.