Kahneman's add one / three exercise In his book Thinking, Fast and Slow, Daniel Kahneman describes a mental exercise which he calls Add One:

To start, make up several strings of 4 digits, all different, and write each string on an index card. Place a blank card on top of the deck. The task you will perform is called Add-1. Here is how it goes:
Start beating a steady rhythm. Remove the blank card and read the four digits aloud. Wait for two beats, then report a string in which each of the original digits is incremented by 1. If the digits on the card are 5294, the correct response is 6305. Keeping the rhythm is important.

This description is a bit vague and I'm wondering how the exercise is supposed to work exactly. He does not mention to cover up the card before reporting the modified digits. This would allow to perform the transformations on-the-fly digit by digit. This does not sound challenging at all and does not fit to Kahneman's following statements of loading the working memory, in particular:

Few people can cope with more than four digits in the Add-1 task.


My assumption was that he means something like that:

Beat 1: Uncover card
Beat 2: Read aloud first digit
Beat 3: Read aloud second digit
Beat 4: Read aloud third digit
Beat 5: Read aloud fourth digit
Beat 6: Wait + cover card
Beat 7: Wait
Beat 8: Say first digit + 1
Beat 9: Say second digit + 1
Beat 10: Say third digit + 1
Beat 11: Say fourth digit + 1


However, a quick Google search reveals a few implementations, for instance:

This blog post resulting in a YouTube video of the exercise.
An Android app slow thinking trainer.

To my surprise both attempts use the dull version of reporting the transformed digits while seeing them.
My question: Is there a better description of the exercise which was originally used in the scientific studies?
I just came across the same section you referenced above in Kahneman's fascinating book. Like you, I was curious how to actually perform it. You are absolutely right, Add-1 (and Add-3 for that matter) seems relatively trivial if you can see the digits while performing the arithmetic.

So I searched for the author's actual study and found a more detailed description of it summarized in another book of his, Attention and Effort (1973). See page 20, which reveals that the experiment was actually harder than described in Thinking, Fast and Slow because the digits were spoken one at time (as opposed to visually seeing all 4 together), thus each digit had to remembered for several seconds until the arithmetic was required.


  The primary task involved the transformation of a digit string: the subject heard a series of four digits (e.g., 3916) at a rate of one digit/second, and he was instructed to pause for a second, then to respond with a transform of that series (4027), adding 1 to each digit of the original set.


I believe a more accurate way to recreate the original experiment with index cards would be to write only 1 digit on each card.