Rolling Bar Defense System
From the Super Mario Wiki
The Rolling Bar Defense System is an obstacle in Mario & Luigi: Superstar Saga. It is only found in the caves under the Relaxation Room, serving as a tutorial for Mario's new Dash ability and Luigi's new Tickle ability. It is a series of spiky tubes that infinitely roll between platforms. Two of these systems are placed just before the red and green Pearl Beans in order to protect them. They can only be bypassed by the brothers using their abilities in the appropriate situation; if either of them comes into contact with one of the bar's spikes, they are sent flying into the air and back to the start.