Because it can use both methods and toggle between them. But determining HR is not instant. You only see HR update every 5s using LED and the first measurement take even longer. So it will keep measuring using LEDs and fall back on that method, when you let go of the crown.
ECG can only be measured using electrodes, so no point having the LEDs running.
Read here how Apple Watch measure Heart Rate
Learn how Apple Watch measures your heart rate, and get tips for a more accurate reading.
Thanks for the article, that helps lot to clarify the problem.
However, in the article of Apple, it says the following:
To use the electrical heart sensor to measure your heart rate, open the Heart Rate app and place your finger on the Digital Crown. You will get a faster reading with higher fidelity — getting a measurement every second instead of every 5 seconds.
This doesn't work on my Watch Series 5. Heart Rate app ALWAYS lights up the green LED, even if I place my finger on the crown. I can't force the app to use electrical heart sensor. Only the ECG app automatically uses the electrical method without firing up the green LED.
So it looks like Apple Watch is using THREE methods to measure heart rate.
2. Green LED
I'm just curious to know as why wouldn't Apple just ditch the green LED if the Infrared seems to be sufficient for the Workout app to measure HR? In the article, it says Infrared only measures in the background when the Watch is not active. I'm guessing that Green LED is simply stronger for reading situation such as workout due to sweat or movement. It's just my thought.
Any thoughts on this?