Sudoku
Forcing net practice
A forcing net is a deeper forcing chain. You still suppose a candidate is true and look for a contradiction, but following singles alone does not get there. The trial has to be carried on with patterns as well, and for longer.
It is the step every Insane puzzle needs, and the hardest thing this site asks of you.
How to spot a Forcing net
Paint the cells a trial fills so you can keep track of it and take it back. Work from a cell with two candidates, and be ready to spot a pair or a locked candidate inside the trial itself.
Practise it on this page
The board above deals positions where the Forcing net is the only way forward, with every pencil note filled in. Cross off the candidates it rules out and press Next. Press Hint to see the pattern marked on the board.
In a full game, Insane is the first level that can ask for it.