Adobe Acrobat 7 Professional - Manual (Page 584)

Adjusting trapping tolerance
Some jobs need only the most extreme color changes trapped, while others require traps
for more subtle color changes. The Step value specifies the threshold at which the trapping
engine decides to create a trap.
To change how much the component inks in abutting colors can vary before causing those
colors to trap, increase or decrease the value for Step in the New Trap Preset dialog box.
The lower the Step percentage, the more often traps are created between colors.