Wedges

A wedge is a move that places your disc on a single empty square between two opponent discs on an edge. Edge discs flip only along the edge. So when both neighbors are opponent discs, the opponent can never flip this disc back. It stays until the end and traps the opponent discs toward the corner, so you can take that edge's corner however the opponent replies.

That is why on an edge you first count the empty squares between the opponent's discs. If the number is odd, you can make a wedge. With one square, wedge in right away; with three, play at one end and let the opponent flip it back from next to it, then wedge into the one square left. If the number is even, the moment you play on one square, the opponent plays right next to it and flips it back, so it does not become a wedge.