from Matt Levine's article linked in another comment "In every deal, there’s an overallotment, which allows the underwriters to sell 115% of the available offer

from Matt Levine's article linked in another comment "In every deal, there’s an overallotment, which allows the underwriters to sell 115% of the available offering to investors, effectively opening a short position. The excess 15% can be purchased by the underwriters in the open market — covering the short position — to support the stock if it goes down. More colloquially, this is known as the “greenshoe.” But in rare cases, bankers will use a strategy called a “naked short,” which allows underwriters to sell shares in excess of that greenshoe portion and then buy them back in the open market to provide even more firepower in the event there is significant selling pressure."