Marcin Wichary, author of Shift Happens and the Unsung essays on software craft, has been cataloging how software fails our hands for years. This new 7,700-word interactive essay has 38 live playgrounds that let you feel the argument in your fingers rather than just read about it.

Wichary’s central argument is that software keeps rediscovering a century of research into how hands actually work: overlapping keystrokes, motor memory, spring-loaded modes, dead zones. For designers, the lesson is that responsiveness at finger scale is not polish. When an interface blocks or delays the narrow timing window our hands rely on, it breaks motor memory and flow.

It turns out, fingers are time travellers. At any given moment, each one is living in a slightly different time – as one finger is moving down to press a key, another is already travelling to the next one, and your brain is thinking of a few keys in advance, visualizing your hands moving to the right place.

[…]

None of this requires touch typing. What we eventually called overlapping happens even if you reside at the awkward intersection of “hunt” and “peck.” Overlapping is a small miracle happening in front of your eyes, and it happens pretty much to everyone.

A capability of our hands and brains to treat our fingers relatively independently, and allow them to move at their own pace without waiting for other fingers (on the same hand) to finish.

The essay walks from 1960s terminal echo buffers through Chrome’s reload-button disabling trick to the moment iOS broke cardinal contracts of programming for the sake of scroll:

[…] competitors reportedly recorded the interactions with a high-speed camera to confirm they were, indeed, running at a constant 60fps.

[…]

iOS stopped running my code in the middle of a function just so it could go back to making sure scrolling was as smooth as possible. No modern programming language would ever dare interrupting your function halfway through; iOS was so dedicated to its user’s fingers that it broke cardinal contracts of programming.

That commitment, prioritizing fingers over execution contracts, is the standard Wichary is asking us to hold in design crits. He closes by turning delight away from decoration and toward restraint:

Sometimes they say that confidence is absence of confidence. […]

In a same way, I think often delight is absence of delight. In places and apps that welcome fast fingers, delight can be abstaining from a transition or something cute but deathly. Pushing for delight of the right kind can mean long fights with frameworks to reduce even one-frame delays…

The interactive format is fantastic. Experiencing painful buffers or re-experiencing how early Mac windows could be dragged around, makes the argument visceral in a way no static screenshot ever could. Start with the playgrounds; they make the timing failures impossible to unfeel.