Sentinels #134

• Region locked to a line. If every open cell of a region sits in one row, that region’s sentinel is somewhere in that row - so no other cell in the row can hold one. Cross the rest of the row out.
• Line locked to a region. The mirror image: if every open cell of a row lies inside a single region, that region’s sentinel is that row’s sentinel, so the region’s cells outside the row can be crossed out.
• Neighbour squeeze. Sentinels never touch, even diagonally. When a row’s candidates are packed into a narrow band, any cell in the row above or below that would touch every one of those candidates can be crossed out.
• Last cell in a row. Every row holds exactly one sentinel. When a row has only one cell left that is not crossed out, that cell must be the sentinel.
• Last cell in a region. Every coloured region holds exactly one sentinel. A region down to a single open cell is solved.
• Last cell in a column. Every column holds exactly one sentinel. When only one cell in a column survives, place the sentinel there.

Every one of these is explained properly in the Sentinels guide.