under $4, page-5

I've got a quick question I was hoping someone may help me answer.

How does short selling decrease a companies stock price???

Doesn't there have to be 2 sides to a short? Therefore someone actually has to be buying the stock?

I thought the only driving factors for stock price are buyers v sellers, if buyers have more power than sellers then price goes up and visa versa - I don't see how someone betting that the stock price will go down, itself make the price go down.

Thanks in advance.