Let's say you want to buy 100,000 shares of PCFG at .017 cents and the Market Maker your broker uses to buy and sell stocks lists shares of PCFG at that price on the Ask. However, he doesn't actually have 100,000 available at the price you placed to buy the shares, he has only 50,000. To do the sale at that price, he sells you 50K shares he has in inventory (or matching a buyers shares and a sellers shares) and 50K he doesn't. He creates 50K to sell to you to make the order or he "naked shorts" to make shares available.
The SEC in all their wisdom (TIC) has allowed the Market Makers and Specialists to sell shares they don't have to someone buying to keep the market fluid. They then have three days to buy those "imaginary" shares back to balance the books.
IMO it simply allows the MM's to cheat and manipulate price movement more easily than simply selling exactly what they have and then moving the price upward. It can and does stall and slow stock runs.