In a race for top-tier consignments, the world's biggest auction houses have offered sellers sweet deals that have chipped away at commissions on some of the priciest offerngs.

Hammer prices have skyrocketed in the last five years for blue-chip art, but the auction house's cut is increasingly going to the consignor in order to secure trophy artworks from the competition.

Bloomberg's Katya Kazakina reports on the rise of “enhanced hammer,” which she describes as "behind-the-scenes deals that are shifting the balance of power inside the notoriously opaque market for art":

Since 2009, art prices have soared and sales have more than doubled. But Sotheby’s commission margins, the revenue from commissions as a percentage of auction sales, have dwindled to about 14 percent from 21 percent. After a roller-coaster ride in recent years, Sotheby’s share price has fallen almost 35 percent over the past 12 months, as of 1 p.m. Tuesday in New York.

Christie’s, which as a closely held company isn’t required to report financial results, has been wielding the enhanced hammer too.