The first half of the video shows the T-Bot being controlled using angular and positional PID loops. The second half uses the same controller but, the coloured disks on top of the T-Bot are rotated by about 40 degrees. Now the T-Bot cannot travel in the direction perpendicular to the line defined by the two discs. While the PID controller is not the most elegant or efficient way of doing this, it certainly proves to be remarkably robust.