Do you mean retarding ignition timing in the tune? If so it reduces torque and increases EGTs but it reduces the likely hood of knocking since it will lower the peak cylinder pressure. But you want to run the most timing you can without knocking, or hitting MBT. MBT is mean best timing, or mean best torque, or minim best timing, or a section of other words that fit the acronym and mean about the same thing. MBT is basically the lower amount of timing advance that results with the most torque possible under each condition. So if you were tuning on a mustang dyno, you would hold the car in one cell in the tune, and keep increasing the timing in that cell and watching the torque increase. Once it stopped increasing you'd pull it down a notch and move onto the next cell. Normally in boost you'll knock before MBT, but its still important to tune to MBT in your out of boost cells to maximize response and fuel mileage.
If you meant cam timing... Well ****, I wrote that for nothing.