A stock with a low float can move much more quickly and powerfully due to the basic laws of supply and demand. If there is a low float, that means there is a low supply of stock to be traded in the market. Therefore, when there is alot of buying interest, but relatively little stock to go around, what happens to the price? Yup, it shoots up. Likewise, if there is a decent number of short positions on a low float stock, a move up in the price will cause shorts to scramble to cover, and with relatively little stock out there to buy, it causes even more upside price action.
The flipside is that a stock with a low float can also move down quickly, since again there is a small number of shares available to trade, thereby making it easier to move the stock's price with less volume.
By contrast, take a look at stocks like CSCO and DELL, which have split and increased their float so many times that it is rare for them to trade in large point swings anymore, since it would take a tremendous amount of volume to move them more than a point or two. This is because their floats are so huge.