https://techcrunch.com/2018/09/19/apple-watch-series-4-review/

Roughly two-and-half minutes into my run, the watch kicks in. There’s a haptic buzz on my wrist.

“It looks like you’re working out,” the watch face reads. That’s followed by a big, yellow button, suggesting I start an indoor run. I tap the neon button and the clock starts, comping me a reasonable approximation of the time it took for the Watch to be sure what sort of activity it was detecting.