One of a couple things can happen while raking and which is going on will depend somewhat on your technique.
First off, by applying light tension and brushing the pick back and forth gently you can often cause the pins to be lifted to the point where they hit the shear line. Since you are applying a very light brushing, not a strong upward force, you will not push the pin stack past the shear line (since it would require additional force to do so, and you are simply not applying enough force). The back and forth motion can cause each of the pins to become set in this manner. This technique has been refered to as raking, though if I recall correctly it is described as scrubbing in the MIT Guide.
Another technique often called raking or ripping is to make a very fast "ripping" motion from back to front (out of the keyway). This motion is sometimes combined with very light tension, or a bouncing motion with the tension wrench. The idea here is to cause the top pins to jump up due to the impact (thus the billiard ball analagy). It will also cause you to scrape a bit of the pins off in the violent action of ripping, so it is a bit more destructive than other methods. Some people seem to prefer to use this ripping technique with a somewhat heavier tension, and though I have not had much success with it, the theory seems to be that it works in a similar fashion to scrubbing. I have found that the violent action of ripping tends to cause me to overset pins.
Hope this clears the matter up some. No doubt there are others who will have more to add on the matter.
Happy picking!
db
