The most essential feature of the Apple Watch Series 9 and Ultra 2 was not available when the watches launched last month. Double tap finally arriving today via the watchOS 10.1 updateallows you to interact with the watch without having to use the touch screen. With a quick “pinch” motion, you can scroll through the new series of widgets in watchOS 10, pause or end timers, skip music tracks, and answer phone calls.

Technically speaking, double-tapping isn’t that new of a gesture. In 2021, Apple introduced Assistive Touch, which is intended for people with limb defects or movement problems. The goal was to give these people a way to browse menus and control the Apple Watch without using their other hand.

At first glance, double-tap may appear to be a recolored version of Assistive Touch, which begs the question of why double-tap isn’t available on older Apple Watches that support Assistive Touch (Series 4 or later, including the first generation SE and Ultra). The short answer is that the Apple Watch Series 9 and Ultra 2 have a more powerful chip. Specifically the new S9 features four neural machine learning engines, which is what powers the double tap. On older watches, assisted touch ran on the main processor.

According to David Clarke, Apple Watch’s senior director of software engineering, this difference is critical. “Because we’re working with a dedicated part of the processor, we don’t have to deal with all the other things the processor is doing at any given time,” Clark says. As a result, the Series 9 and Ultra 2 watches detect the double-tap gesture 15 percent more accurately, and the feature itself is much less power-hungry.

For double-tap to work, the device has to process a huge amount of data. At the most basic level, the algorithm that detects the double-tap gesture is trained on data from the accelerometer, gyroscope, and optical heart rate sensor collected from the wrist.