https://www.techradar.com/news/apple-watch-4-teardown-reveals-a-few-new-parts-and-redesigned-interior

It made sense to peer into the iPhone XS first, but now we’ve gotten an inside look at the Apple Watch 4. An iFixit teardown has shown us that while there are only a couple new elements, Apple has shifted around the smartwatch’s internal parts to eke out a bit more sound and battery.

The biggest addition is the new pair of sensors, which operate as electrodes allowing the smartwatch to take ECGs. The first is a conspicuous gray ring on the bottom of the watch, while the other is within an updated digital crown, which the user squeezes with their other hand during a reading. The watch measures voltage between the two to take an ECG.