It's the only manipulative practice allowed by the SEC because of the need to "maintain a fair and orderly market". This article describes it fairly well in its abstract: http://economics.ouls.ox.ac.uk/10713/1/IPOstabilization.pdf.

"Stabilization is the bidding for and purchase of securities by an underwriter immediately after an offering for the purpose of preventing or retarding a fall in price. Stabilization is price manipulation, but regulators allow it within strict limits - notably that stabilization may not occur above the offer price. For legislators and market authorities, a false market is a price worth paying for an orderly market."

EDIT: Just included the full description.