Okay, say you store 2 in up.
First this impulse is scaled back by the movement factor in the physics page. The default is .8, which means you must multiply 2 by .8, which gives you 1.6
Next, this acceleration is divided by mass. Mass = 0.5 + (.body / 10000) + (.Shell / 200).
Then it's applied to the bot.
If a bot is moving, friction acts on it, slowing it down by percentages each cycle.