https://www.cnet.com/news/apple-watch-4-teardown-reveals-how-the-ekg-probably-works/

The new S4 processor which drives the whole Watch now pops out easily; Apple's practice has historically been to glue down many of its processors. Beneath it are the charging coils and a magnet in the middle.

When you press the digital crown, it measures the opposite hand and closes the signal between it and the wrist electrode. Then it all gets funneled into the new S4 chip.