From section 5.10...
"When riveting a lap joint, the expansion of the aluminum caused by the setting pressure and the expansion of the rivet causes the overhanging edge of the sheet to bend upwards slightly. This creates a number of problems, like making the lap joint appear wider than it is, making painting more difficult, etc. One easy method of minimizing this effect is by pre-bending the last 1/4 inch of the skin downward just a small amount before it is clecoed and riveted. When the rivets are driven, the skin will be flattened and the pre-set in the skin edge will tend to hold it flush. Except for very rare circumstances, the amount of bend (break) is very small and when done properly is almost undetectable with your eye. The goal is to do just enough to keep the skin lying flat but not too much or it will be obvious that it was done."