The noise at the tip is milder than a firecracker, but the tiny flicker at the end makes it seem otherwise. Sixty feet away from the tennis courts at Dolores Park, the popping is audible over the ball-bashing and the accompanying grunts. Minutes before 8 a.m. on a recent Friday, Hannah pulls one whip after the other from her pink tote bag. Just a warm-up, she promptly clarifies.

Among the many parks in San Francisco, she chose Dolores because of its art community, booming every Sunday on the green plateau. “It’s just very welcoming, lots of flowers, people with their dragon staffs, fire spinners, flags, dancers … ” It’s a solo dance this morning, as she waits for her first student of the day. More than a bicep stretch, the whip-crack looks like a warm-up for the whips themselves.

The flicker at the end is quite hard to capture on video, but it is more than a trick of the eye. The loop travels fast enough to break the sound barrier; what we’re seeing are diminutive sonic booms, one after the other. Hannah stops to talk only so she can spend some time re-tying the “popper” at the end of the whip.

“It is the popper that makes the noise; delicate, but very powerful.”

She settles for the pink pair, as she whips both simultaneously. “It’s less intimidating,” she adds as her client finally arrives.