Glue is the technique for a filled cell that sits near a boundary without touching it. The run that covers the cell can still slide a little, but the boundary limits how far — and every legal position must keep covering the glued cell. The intersection of those positions fills cells; the union crosses others off. It is the overlap method again, but localised around one known cell, which makes it much stronger.
Concretely: a width-8 line with the single clue 4, and the second cell already filled. The run must cover cell 2, so it starts at cell 1 or cell 2 — nothing else reaches. Both positions cover cells 2, 3 and 4, so those fill. No position reaches past cell 5, so cells 6-8 are crossed off:
The rule of thumb: a filled cell at distance d from a wall, covered by a run of length k, forces k − d cells to fill on the far side of it (counting the glued cell itself), whenever that number is positive. The closer the cell hugs the wall and the longer the run, the more you get. At d = 0 — the cell on the wall itself — glue degenerates into plain edge logic and the entire run resolves.
Remember that your own marks count as walls. A filled cell sitting one space away from an X you placed earlier is glued exactly as if it sat near the grid edge. This is why diligent marking pays compound interest: each X you prove turns nearby filled cells into glue candidates, which fill more cells, which give the crossing lines new anchors. Scan for glue immediately after any marking pass — it is the cheapest follow-up deduction on the board.