The flatland model highlights the basic defect that enables lock picking to work. This defect makes it possible to open a lock by lifting the pins one at a time, and thus you don't need a key to lift all the pins at the same time. Figure 4.3 shows how the pins of a lock can be set one at a time. The first step of the procedure is to apply a sheer force to the lock by pushing on the bottom plate. This force casued one or more the of pins to be scissored between the top and bottom plate. The most common defect in a lock is that only one pin will bind. Figure 4.3a shows the left pin binding. Even though a pin is binding, it can be pushed up with a picking tool, see Figure 4.3b. When the top of the key pin reaches the sheer line, the bottom plate will slide slightly. If the pick is removed the driver pin will be help up by the overlapping bottom plate, and teh key pin will drop down to its initial position, see Figure 4.3c. The slight movement of the bottom plate causes a new pin to bind. The same procedure can be used to set the new pin.
Thus, the procedure for one pin at a time picking a lock is to apply a sheer force, find the pin which is binding the most and pish it up. When the top of the key pin reaches the sheer line, the moving portion of the lock will give slgihtly, and driver pin will be be trapped above the sheer line. This is called setting a pin.
Chapter 9 discusses the different defects that cause pins to bind one at a time.
Table 4.1: Picking a lock one pin at a time.