Apple Watch Double Tap and Wrist Flick gestures only work on newer watch models for a reason.

Tim Hardwick from MacRumors:

You may have missed it, but select Apple Watch models are gaining a new Wrist Flick gesture in watchOS 26 that allows users to dismiss notifications and return to their watch face with a simple wrist movement.

The gesture works by turning your wrist over and back, using the accelerometer and gyroscope to detect the motion. With the gesture, you can dismiss incoming notifications, mute calls, silence alarms and alerts, and return to the watch face without needing to use your other hand.

Wrist Flick joins the existing Double Tap gesture as another one-handed control option for Apple Watch users. The feature can be toggled on or off in Settings ➝ Gestures ➝ Wrist Flick.

The new gesture is only available on Apple Watch Series 9, Apple Watch 10, and Apple Watch Ultra 2 – the same models that support the Double Tap gesture.

Apple doesn't say exactly why older models don't support the feature, despite having the same sensors, but it does use a new machine learning model, suggesting that only the newer chips found in recent Apple Watch models are powerful enough to handle it.

Apple explained how Double Tap works on Apple Watch Series 9 (S9 SiP) in their Fall 2023 Keynote, and I’m assuming the same reasoning applies to the new Wrist Flick feature:

This new gesture is enabled by the powerful neural engine in Series 9, which processes data from the accelerometer, gyroscope, and optical heart sensor in a completely new way.

It uses a machine learning algorithm to detect the unique signature of tiny movements and changes in blood flow when your hand and fingers perform a Double Tap.

Apple is usually good at backporting software features, but this time it’s hardware related.