When it comes to taking heart rate readings, Apple Watch is pretty accurate. However you can make your heart rate readings even better and increase the speed at which they are taken on your Apple Watch. This method works on all Apple Watch Series 4 devices regardless of your location or where you purchased the Watch from.
Since the beginning of the Apple Watch users are able to take heart rate readings by launching the Heart Rate app. When you launch the app the Watch uses its infrared light-based sensors to record your readings from the wrist.
However starting Watch Series 4 Apple has added another way of taking heart rate readings. Now users can place their finger on the Digital Crown to record better and faster readings. The Watch detects when the user has placed his or her finger on the Digital Crown, and switches from infrared light-based sensors to the electrodes found on electrical heart rate sensor at the back of the Watch and the Digital Crown.
The readings taken while the finger is placed on the Digital Crown are more accurate and taken with higher fidelity since the Watch records the reading every second instead of every 5 second as with the traditional method.
This is not to say that you cannot take heart rate readings without putting your finger on the Digital Crown, you still can however using this method provides improved readings.