Breathing Emergency Detection watches for blood oxygen dropping to a dangerously low level. If that happens and the wearer then stops responding, the watch calls emergency services and the wearer's emergency contacts on its own. Google introduced it with the Pixel Watch 5 on August 12, and a day later the Pixel Watch 4 is getting it too, through a software update. Before it calls anyone it confirms you have not been asleep for 30 minutes, because sleep apnea can look like the emergency it is watching for.
Nobody in the United States can use it on either watch, because it has not been approved there. It works in nine countries and all nine are European: the UK, Austria, Denmark, Germany, Ireland, the Netherlands, Sweden, Norway and Switzerland. Insulin Resistance Trends and Blood Pressure Trends are the other two Health Guardian features from the same launch. Google gave no date for a US launch and no list of which regions come next.