The pencil analogy is OK, but you're using the wrong term to describe it. Precession refers to the circular change in the axis of a rotating body such as a gyroscope. It describes the wobble observed in a spinning top or coin, and has nothing to do with why ball bearing pedals and British bottom brackets are threaded the way they are.

The key to understanding why pedals are threaded opposite to what you'd expect is to look closely at how ball bearings act. Draw yourself a quick sketch consisting of a circle surrounded by smaller circles and one larger circle around the whole thing to represent a ball bearing. The outer ring is the pedal body race, and the inner circle is the spindle with the other circles being the balls.

As you pedal forward, the right pedal body rotates to the left with respect to the right crank arm. Going to the sketch draw arrowheads on the outer ring indicating counterclockwise rotation. Since the balls have a rolling (not sliding) motion in the race they each rotate to the left. (draw in the arrows). Now extending the rolling motion to the innermost circle you'll see that it's driven to the right, or in the opposite direction of the pedal body. The effect is mirrored on the left, and is the same in BB, except that there the spindle is the input.

BTW- it's important to note that this happens only ball bearing pedals when things are OK. Bushing pedals should be threaded in the opposite direction, and if BB pedals have rust or frozen bearings they'll unscrew.