The ERIC robot has two wheels but no caster wheel, so when it changes velocity the circuit board, motor and battery pack produce a rocking motion, which can be reduced with simple control.
The animation shows the robot. You can increase or decrease the speed, reverse direction, or set the robot on to a slope: all these velocity changes cause rocking.
See what happens when you change speed. Note the difference when you reverse direction at faster speeds. See also what happens when the robot goes on or off the slope.
Turn control On and investigate the difference.
Press U and D to speed up or down, R to reverse motor, S to toggle whether on slope, C to toggle whether Control is used, and V to toggle 3D/2D view.