Boxes and lines share cells. That overlap is a secret weapon.
Pointing (a kind of Locked Candidate) happens when, inside one box, all remaining places for a digit sit on the same row (or same column). The digit is “pointing” along that line.
Why it works
The box still needs that digit. Every leftover candidate for it is on one shared line. So wherever the digit finally lands in the box, it will be on that line.
Therefore the digit is already “booked” for that line *inside* the box. It cannot also appear on the same line *outside* the box — that would put two copies of the digit on one line.
You are not placing the digit yet. You are defending the line from a double-booking.
Demonstration
A box must still contain every digit 1–9. Look inside a box for one digit and mark every empty cell where it can still go.
Here 2 only appears on one line inside the box. So the box’s 2 must live on that line — the possibilities shrink to that shared line.
That means the line already has a future 2 coming from this box — it just has not been placed yet.
So remove 2 from the same line outside the box (red). Those cells would create a second 2 on the line.
Example 1
In a box, find a digit whose candidates all sit on one row or column.
2 is locked to that line inside the box.
Eliminate 2 from that line outside the box (red). The box already reserved the digit for that line.
Example 2
In a box, find a digit whose candidates all sit on one row or column.
5 is locked to that line inside the box.
Eliminate 5 from that line outside the box (red). The box already reserved the digit for that line.