Medium Sudoku
One technique separates this board from the easy one. Somewhere on the grid you will reach a point where no single cell can be settled on its own, and you have to turn the question around: instead of asking what can go in this cell, ask where in this box the number seven is still allowed to go. If the answer is one cell, that cell is a seven, however many other digits it could also have taken.
Where the hidden single lives
Take a box with four empty cells. Consider only the digit 7. Three of those cells sit in rows or columns that already contain a 7 somewhere else, so the 7 cannot go there. One cell is left, and it must be the 7 – even if that cell could equally have held a 2, a 5 or an 8 as far as its own peers are concerned. The digit is hidden because the cell does not look decided; it is single because the unit has run out of places to put that number.
It is a different search, not a harder one
A naked single is found by looking at a cell and reading off what surrounds it. A hidden single is found by looking at a unit and a digit together and eliminating the places that digit cannot go. Those are different motions of the eye, and that is the real reason people stall on this grade: they keep scanning cells when the board has stopped rewarding it. When cell-by-cell reading dries up, pick a digit that appears five or six times already and walk it through the boxes it is missing from.
What the sudoku medium label is measuring
The engine solves each candidate puzzle with a fixed ladder of techniques, always taking the cheapest move available, and records which rungs it had to use. A board reaches this page when it needed at least one hidden single and never needed anything above that. Every puzzle here is therefore finishable with two ideas in total, applied in whatever order the grid offers them.
Two hundred and forty-three questions instead of eighty-one
Here is why this grade feels like more work when the deduction itself is no harder. A naked single asks one question of each cell – what is left for this square? – and there are eighty-one squares, most of them answered at a glance because the cell is already full. A hidden single asks about a unit and a digit together: where in this row can the 7 still go? Twenty-seven units and nine digits is two hundred and forty-three questions, three times the sweep, and not one of them can be settled by looking at a single square.
That ratio is the honest reason people stall on a medium board. It is not that the grade demands more cleverness than the easy one. It is that the cheap sweep has stopped paying and the expensive one has not started, and the gap between those two feels from the inside exactly like being stuck.
If it is coming out too fast
The step up from here is not more blanks, it is a third technique: on hard, two cells that share the same two candidates start locking digits away from everything else in their unit, and the board stops being solvable one cell at a time. Going the other way, easy restricts itself to cells that can be read in isolation.