Loop a length of fencing wire around one fence post just above the crossbar. Loop it around the other post at a point at least halfway between the top of the post and the ground.
Tie one end of the wire into an eye loop, tying it off with six or more tight turns of wire. Cut off any excess wire below the last turn.
Pull the other end of the wire through the loop and tie it off with six or more tight turns of wire. Cut off any excess wire below the last turn.
Insert the twitch post between the two wires. Choose a point at which you can rotate the twitch post through a full circle perpendicular to the fence line, but close enough to the top that you can brace the twitch post against the crossbar without bowing the wire.
Twist the wire using the twitch post as a handle. Continue twisting until the wire is tight between the two fence posts.
Place the end of the twitch post against the crossbar so that the tension of the twisted wire helps hold it in place. Nail the twitch post to the crossbar.