Regulatory clearance is required for the ECG feature, so it's only launched so far in the United States, Hong Kong, Canada, Singapore, and most countries in Europe, including the United Kingdom.
The feature is said to already be live for users in India today if they own the Apple Watch Series 4 and upgrade to watchOS 6. It will also be available for Indian users who purchase the Apple Watch Series 5, available in the country from September 27.
ECGs are captured by holding a finger on the Digital Crown of the Apple Watch, which will detect a sinus rhythm (normal), an abnormal result, or, sometimes, test results will be inconclusive.
The idea is that you can take this information to your doctor, if the results are abnormal, and use it as a basis to discover if you have any larger issues at hand. In total, it takes about 30 seconds for the Apple Watch's ECG functionality to complete.