What this mode is
Ordinary sudoku never hurries you: you decide where to look and how long to think. Here that choice is partly taken away. One sector out of nine goes live, a countdown appears on it, and while it runs you know exactly what is at stake.
One thing matters more than any other: the highlighted sector is what is being scored, not where you should be working. In sudoku, information arrives in a sector from the rows and columns crossing it, so you will almost always have to open cells outside it first. We measured this across 120 boards: only about a fifth of the cells you must place before a sector closes actually lie inside it.
The timer is not invented. It is derived from the techniques the board genuinely demands at that moment: a naked single costs less, a pointing pair costs more. The same sector on two different boards will get two different clocks.
If you run out of time the sector closes itself and fills with the correct digits. The run does not end: the board stays solvable and only the point is gone. At the end you see how many sectors you saved out of nine.
A wrong digit costs ten seconds and a hint costs fifteen. There are no lives here: the clock is the only thing that counts.