Apple-designed its Apple Watch originally to help the user track their health, and work on their fitness. Due to this, there are a few features embedded in the Apple Watch that will help in executing this core function.
Among them is an accelerometer. Apple Watch uses an accelerometer to identify movement.
How does your Apple Watch know when you stand?
The embedded features like the accelerometer are basically how your Apple Watch can detect when you stand and how many steps you take. The motion sensors make this identification even more accurate.
When you move your arms as you change from a seated position to a standing position, this helps the Apple Watch identify that you’ve stood up.
When your Apple Watch fails to register when you stand up, then it means you may need to troubleshoot your device. It could also mean that your apple watch is not touching your wrist so you may need to turn on Wrist Detection.
Again, it could mean your Fitness Tracking is disabled so you may need to Enable it.