Locked candidates (claiming)
A digit’s places in a row or column all lie in one box, so it’s removed from the rest of that box.
A technique for Medium puzzles. Play a Medium puzzle
What it does
The reverse of pointing. In a row or column, look at where a digit can go. If all those places are inside one box, the line’s copy of the digit is in that box, so the rest of the box can’t have it.
How to spot it
Look for a row or column where a digit has only two or three candidate cells, close together in one box.
What it lets you do
You remove the digit from the box’s other cells, the ones outside that row or column.
Try it
Find the Locked candidates (claiming) on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
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.