Getting the puzzle ready for you…
How to solve sudoku 2 of 6
A box shares three squares with a row and three with a column. If a digit within the box can only go in those shared squares, you know something about the whole row — and the other way around. This is the first technique where you cross out candidates instead of placing digits.
Within one box a digit can only go in squares that happen to lie in the same row (or column). Where in the box it goes you don’t know, but that it goes in that row you do — so outside the box that digit drops out of that row.
If the 4 in box 1 can only go in the top row, the 4 of that row is surely in box 1, and so no longer in box 2 or 3.
The same idea, the other way around: in this row (or column) a digit can only go in squares that all lie in one box. The row then claims that digit for that box, and it drops out of the rest of the box.
If the 4 in row 5 can only go in the three squares that lie in box 5, then the 4 of box 5 is in row 5 — and so not in the other two rows of that box.