Apple tested many [Taptic Engine] prototypes, each with a slightly different feel. “Some were too annoying,” Lynch says. “Some were too subtle; some felt like a bug on your wrist.” When they had the engine dialed in, they started experimenting with a Watch-specific synesthesia, translating specific digital experiences into taps and sounds. What does a tweet feel like? What about an important text? To answer these questions, designers and engineers sampled the sounds of everything from bell clappers and birds to lightsabers and then began to turn sounds into physical sensations.

If you haven't read it yet, please do. Brilliant and beautiful composed article on Apples latest.