Mihaly Csikszentmihalyi spent years asking rock climbers and surgeons about the stretches where the work stopped feeling like work. What came back was not about the work. It was about the fit between what the thing demanded and what the person could do that day.
A fit is a ratio and a ratio has two ends. Make the task harder and the same person tips into anxiety. Leave the task exactly where it is and let the person get better at it, and the same task tips into boredom. Neither of those is a property of the task.
Below is a keypad with a code to enter and a clock. The difficulty slider sets how many digits there are and how many seconds you get. The skill setting is a rate — how many digits a second somebody at that level enters. The dot sits where those two meet, and the band is drawn where the ratio between them falls between 0.7 and 1.4.
The rates are made up. So is the width of the band. They are there so the arithmetic moves and you can watch the dot leave the band from either side. There is no version of this where ten seconds on a web page tells you whether you were in flow.
Hold the skill setting still and move difficulty on its own. The dot runs up and down one dashed line. The band does not move, the line does not move, and the dot leaves the band in two directions for two entirely different reasons.
The two sliders need JavaScript. What is drawn is difficulty 5 against somebody who has done it a few times.
Seven digits in 6.5 seconds asks for 1.08 a second. This person does 1.05. The ratio is 1.03, which is inside the band that runs from 0.7 to 1.4.
Why the band moves under you
Push the skill setting one notch to the right. The dashed line moves with it, and the difficulty that was sitting in the middle of the band is now underneath it. Nothing about the task changed. Nobody touched the product.
That is the whole reason an onboarding flow that was correct in week one is tedious in week four, and it is why the usual fix does not work. Making it clearer makes it easier, and easier is the direction it was already too far in.
It also runs the other way, and faster. A person who has done something eleven times and then meets a redesign is back at the left of the plane with no warning, and the thing that was comfortable on Monday is asking for more than they have on Tuesday.
What it costs you
One difficulty serves one skill level. A single fixed setting sits in the band for one slice of the people using it and outside it for everybody else. That is not a failure of the setting. It is what a fixed number does against a spread of people.
Anxiety and boredom leave the same trace. Both of them show up as a person who stopped. The session length is similar, the drop-off point is often the same screen, and the two fixes are opposite. Guessing wrong makes it worse rather than neutral.
You do not get to pick which end somebody arrives from. The same screen is the first one for a new person and the four-hundredth for somebody who works in it all day, and both of them are looking at it right now.
Difficulty is not the only dial on the plane. The other axis is capability, and it moves with a good default and with an undo. Raising what somebody can do is usually cheaper than lowering what the task asks, and it leaves the task intact for the people who were fine.