Locked candidates (pointing)
A digit’s places in a box all lie on one line, so it’s removed from the rest of that line.
A technique for Medium puzzles. Play a Medium puzzle
What it does
Inside a box, look at where a digit can go. If all those places lie in the same row or column, the box’s copy of the digit must land on that line. So no other cell of that line, outside the box, can have it.
How to spot it
Look for a box where a digit has only two or three candidate cells, and check whether they share a row or a column. Notes make this much easier to see.
What it lets you do
You remove the digit from the cells of that row or column outside the box. Pointing never places a digit by itself; it clears the way for a single.
Try it
Find the Locked candidates (pointing) on this board. Pick the cells it clears, mark the candidates it removes, then check.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Medium level; any Medium puzzle may need it too.