Loop Avoidance

Medium Used by Smart Hint

Drawing this edge would close a loop right now. If closing it here can't be the whole solution, it's ruled out — but if closing it here does satisfy everything, it's the final edge, so it's forced in.

Edge on the loop
Edge the technique adds
Edge ruled out
Cells that establish the pattern

How it works

Three sides of the center cell are already lines, forming almost a complete small loop around it.
Drawing the last side (the bottom edge) would close that loop right here.
Checking the rest of the board: with every other undecided edge left blank, every clue still works out — so this small loop CAN be the entire solution.
That makes the bottom edge forced in — draw it as a line.