Trophy description - Find the secret route while on a diet.

Trophy - Bronze

You will unlock this trophy in in Act V, Curtain II. During this level will be inside a fat boys stomach rolling through the level. You need to cut the Calirbus by pressing the Square button to pick up speed and press X to jump over the various hazards. Towards the end of this section once you jump up the falling platforms will come to a part with large caterpillars on the ground between pots of fire. You need to jump off the first one by pressing x to enter the secret passage above. If you miss it just lose all three of your heads by jumping on the fire and you will restart at a previous checkpoint. This saves having to restart the whole level again. The trophy will unlock once you enter the next section.

See the below video by Wozamil for the exact location of the secret route and how to get there.