Pump will make the price rise, dump will make it fall. I believe.
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
See it as a supply and demand thing. If you dump you will increase supply thus lowering price.
Thats right, pumping and dumping takes a lot of resources however to really move the price on gox. The idea is to knock out peoples limit orders to buy or sell and profit on the manipulation caused.