Growing effect

http://www.liquideye-designs.com/main/index.html

i’ve seen this effect on alot of sites and i would like to know how it’s accomplished…where an animation basically draws itself onto the stage…and when another link is pressed it retracts everything it did …

1.how is that effect done?
2.when the effect is retracting is that done manually or is it script that tells it to go backwards?? (i dont know so maybe thats why the question seems a lil weird)…