Edward C. Baig, USA Today:

Apple and Johnson & Johnson are teaming up on a study to determine whether the latest Apple Watch, in conjunction with an app from the pharmaceutical company, can accelerate the diagnosis of a leading cause of stroke.

Atrial fibrillation, or AFib, is an irregular and often rapid heart rate that causes about 130,000 deaths and 750,000 hospitalizations each year in the U.S., Johnson & Johnson said. Up to 30 percent of cases go undiagnosed until life-threatening complications occur. Worldwide, about 33 million people have the condition.

And:

Burton believes “the study has the potential to show that there is a lot more atrial fibrillation out there in the real world in older people than we ever imagined, and if you use a tool like an Apple Watch to detect and funnel people to care, you can really drive down stroke risk in those patients.

So what does AFib have to do with stroke? From this article from the National Stroke Association:

Atrial fibrillation (AFib) affects an estimated 2.2 million people in the U.S. AFib is a type of irregular heartbeat, often caused when the two upper chambers of the heart beat unpredictably and sometimes rapidly. These irregular heartbeats can cause blood to collect in the heart and potentially form a clot, which can travel to a person’s brain and cause a stroke.

I do get how the Apple Watch can detect AFib. Not clear exactly what additional role the Apple Watch will have in predicting/preventing stroke. But that’s what the study’s for.