Quote Originally Posted by brian steinberger View Post
Pre-flashing eliminates the papers inertia. So any light to hit the paper after pre-flashing will produce a tone.
And inertia can be described as a 'threshold' of sorts. Light can strike the paper surface, and you develop it and there is no tone. The threshold means that a certain amount of light has to strike the paper surface before the paper develops a tone in the developer. That's inertia; like pushing a square box forward on a surface - if you apply a little bit of force it may not move, but if you apply more force it will.

Edit: Michael is correct. I was typing along...