This article is an explanation of Auto Trim Hopper.

This feature is located under the Campaign Settings under the Hopper Tab. 

To the right of the Auto Trim Hopper toggle, it says:

This feature automatically removes leads from the hopper in excess of the Hopper Level. This will not affect Lead Recycle, List Mix or New Leads inserted in Lead Call Order (2nd New, 3rd New etc..)

The purpose of this feature is to limit the hopper's leads to the current hopper level. The system will compensate the hopper's leads when filling by adding more when the hopper is almost empty. In the case of the screenshot, the hopper's level is at 1000. When using the system, your hopper leads can go above 1000 based on other factors such as lead recycle, API inserted leads, etc.

By setting the Auto Trim Hopper on, you will limit the leads in the hopper to the hopper level.