Quoting David Tutwiler in an Amazon.com review:

This cartridge will disable your printer once it (the cartridge) tells your printer it’s empty. The cartridge has a window that is closed (blocked) by toner. This window runs through the cartridge from one side to the other. When the toner is full, the window is blocked, when the toner is seemingly empty, a laser is able to shine through the cartridge one side to the other thus signaling to the printer to shut down for cartridge replacement. I have found that when you obstruct this window with a simple piece of masking tape, you will have at least 33-50% more toner left.

Worked here in the office on our Brother DCP-8065DN laser printer with a TN580 cartridge. Your mileage may vary, of course.