From the Super Mario Wiki
The Luiginary Speedometer is the fifth Luiginary Work seen in Mario & Luigi: Dream Team. It allows Dreamy Luigi to speed up or slow down time. It is activated and used by Starlow by grabbing either part of Luigi's mustache or his nose on the touch screen.
Speeding up time allows Mario to run against the direction of Conveyor Belts. Slowing down time allows Mario to jump much further than usual. This Luiginary Work is also useful in many timer challenges in which Mario must hit an ! Block within a certain amount of time.