When I first saw the video that hooked me, I assumed it was a hoax.

“Improvised Partner Dance,” the overlay text said, along with: “Random Partner & Song.” The song was the 2007 chart-topper “Forever” — wedding DJ music, the kind of track that might get both grandparents and grandchildren on their feet. The partners — Emeline Rochefeuille and KP Rutland — moved with impossible fluidity, playfully connecting their movements to the rhythm and the familiar lyrics.

I couldn’t quite place the dance style. There were partnered turn patterns I recognized from mambo, flicks of the leg that seemed borrowed from Argentine tango, a moonwalk, the running man. But as I watched, mesmerized, I felt sure of one thing: No way did they improvise this.

Then I saw that the video — which has more than 20 million views on TikTok — was hashtagged #westcoastswing.

This was … swing dance?

A recent wave of viral videos has been spurred by this kind of two-pronged disbelief. The West Coast Swing clips are often filmed during competitive “Jack and Jill” events, at which dancers are paired randomly and do improvise together to a random song, however unconvinced viewers might be. (“I had a commenter say the other day, ‘If you believe this wasn’t rehearsed, I have a bridge to sell you,’” Rutland said in an interview.)

West Coast Swing is a descendant of the Lindy Hop, the swing dance that originated in Harlem in the 1920s. But today’s West Coast Swing has few of the markers traditionally associated with swing dance: not much big-band jazz, little of the Lindy Hop’s irrepressible bounce. The music selections are broad, the dance’s smoothed-out standard steps are friendly to interpolation, and the dancers are often fluent in a range of other styles. Participants in a high-level West Coast Swing Jack and Jill can seemingly lead and follow each other anywhere — right across song and dance genres.