What does the Apple Watch side button do?

The Apple Watch's Side button serves several purposes depending on the gesture applied and the current version of watchOS:

• Show or Hide Control Center: A simple press of the Side button displays or dismisses the Control Center, allowing quick access to various settings and toggles.
• Access Apple Pay: Quickly access payment methods on Apple Pay by performing a double-press of the Side button.
• Activate Emergency SOS: Press and hold the Side button to initiate this function, enabling you to contact emergency services directly from your watch.
• Personalized App Launcher: Customize the Dock to access preferred apps instead of recently opened ones. Set up in Watch app > My Watch tab > Dock section within iOS.
• Poweroff & Restarts: Long press the Side button to power off or reboot the watch.
• Control Center Behavior Change: Starting with watchOS 10, a single press of the Side button now triggers the Control Center; previously, another part of the user interface managed this action.
• Force Quitting Frozen Apps: While an active app is frozen, long press the Side button, wait for the power options menu, then repeat the long press to quit the problematic app.
• Legacy Usage Changes Over Time: Initially intended for sending heartbeats (Friends feature), functionality evolved along with updates, and newer devices like the Apple Watch Ultra offer alternative UI interactions for certain tasks.

Keep in mind that your experience with the Side button may vary based on the watchOS version and specific Apple Watch model you own.