Pick a digit and a 3×3 box that does not contain it yet. Look at the rows and columns crossing that box. Every row or column that already holds the digit eliminates all three of the box's cells along it.
Cross off enough lines and often only one empty cell survives. That cell must hold the digit — not because you worked out what else could go there, but because the digit has nowhere else to go.
This is the technique most players use without knowing its name, and it is what "scanning" means in practice. Working digit by digit rather than cell by cell is what makes it fast: one digit, nine boxes, and you have swept the whole grid.