The $44.6 million stegosaurus, the most valuable fossil ever sold at auction, wasn’t the only thing that caught the attention of viewers mesmerized by a 15-minute bidding war at Sotheby’s on Wednesday.

At the helm of the gripping sales battle was Phyllis Kao, a spirited auctioneer who appeared to delight in the volley, helping to draw in millions of curious online spectators.

Around 10 a.m., Ms. Kao, a vice president of client strategy for Sotheby’s, started the bidding at $3 million with a cool lean on the podium.

“Emily draws blood first,” Ms. Kao says of an early bid of $3.8 million in a captivating video.

Wearing a gray Armani jacket, with her dark hair twisted up and pinned back in a pony tail, Ms. Kao supported herself on her forearm and steadied her gaze toward the bidders like an opponent challenging a rival. Seven bidders held phones to their ears, relaying offers for the bones from buyers around the world.

“At $6 million, thank you, Simon Shaw,” she says, with no qualms about calling on bidders by name, making them feel seen, and special. “At $6 million, nice way to start.”

But the alchemy is in her stance.

Ms. Kao shifts her weight casually from foot to foot, tipping her body forward and back. She bends toward the packed room with a gleam in her eye, almost flirting with the bidders. Then she rocks back to show that, maybe, actually, she doesn’t care about their bid at all. Aloof, she subtly feigns indifference.