I should expound a little more...
What I mean is:
Impulse is acceleration times mass. In older versions, the program would automatically worry about how hard you had to push to get going at some desired acceleration depending on your mass.
In early versions of 2.4, the program didn't do this for you. How fast you actually went depended on your mass, so smaller bots tended to 'jerk' around because they were using too much effort.
Sort of the 'strap a jet pack on a gold fish' dilemma. ;)
In newer versions of 2.4, you'll need to add "use NewMove" at the top of a bot's DNA to see this effect.