Thwomp Stompers are an item used in the Super Mario Bros. film. They are a pair of large boots which propel the user into the air. They are powered by small cartridges that resemble Bullet Bills which are inserted into a slot in the heel of the boot.
The boots are first seen used by Big Bertha after she steals Princess Daisy's necklace from Mario and Luigi at the beginning of the film, but are referenced earlier in a shot of the city that displays a store that sells them. Much later, Big Bertha allows Mario and Luigi to use her Thwomp Stompers to escape from Lena and her Goomba army at the Boom Boom Bar. They are later used once again at the end of the film by Mario to knock Koopa into an empty vat above the streets of Dinohattan, then by Luigi to evade Koopa's Fireballs.
Artwork for the Thwomp Stompers reveals development towards the item seen in the released film. The first (left) is a basic concept that has yet to develop the "Super Mario" feel to it. The second (right) is much closer to what is seen in the finished movie, but the interior and exterior colors are swapped.