https://brighterimagelab.com/sp_faq/lab-direct-teeth-whitening-process-remove-stains/

Our Hi-Intensity™ Teeth Whitening Gel removes most stains with a gentle action that whitens your teeth. As the active ingredient in the gel (carbamide peroxide) is broken down and oxygen acts upon the stained organic deposits, which oxidizes them into a white color, making the overall tooth color closer to its natural unstained color.