In the final Phase Shift course, the one that is timed, you can trigger the cutscene that plays when you complete it while still Phase Shifting - by shooting straight upwards from the last purple barrel, I crossed the platform while missing the ending Phase Plate. This triggered the cutscene that plays for the first 'successful' finished run after the timer is added, though it is glitched with the camera in the wrong position. After the cutscene finished, the Phase Shift continued upwards into the wall, hitting in and resetting me to the initial Phase Plate. However, this did count as completion of the course, and after I made my way through the course again, both timed chests were available. That's the 'exploit' part, getting access to those chests without properly completing the course.