Basiclly, it involves trimming the “snorkel” down to allow more of an airflow. This process has been documented on the Forum at Snorkel Mod Guide

There two reasons why it should be left, in the opinion of — YTC1 A great deal of work is performed by engineers and technicians in designing airflow and noise.They don't just do it do the fun of it. On my own bikes I have found it made little or no difference. Using an Iridium plug and a K&N filter is the better option.

The second reason, when you trim the snorkel top, you have less protection for dropping small items down into the air box.