According to Bloomberg, one way Apple could implement an EKG is by having users hold the sides of the Watch, which would then send a weak electrical current to their heart. Of course, as with any potential new feature though, it could work differently once it's finalized. There's also a chance that Apple gives up on implementing the feature entirely. But given just how useful it could be -- better tracking could help people prevent strokes and heart failure ahead of time -- that seems unlikely.