As the needle traces the record grooves, it will start to drift inward very slowly. Then as the needle follows the record inward, the arm will start to angle inward. When the arm angles inward by a small amount, it triggers a switch (I think it's a solenoid switch, can't remember), which then causes the carriage motor to move the whole tone arm carriage inward slightly, thus straightening the arm again so it's perpendicular to the record again. I believe Peter was suggesting that if this isn't adjusted properly, then the arm may be reaching too far of an angle before triggering the carriage to move it over again.
Austin