Once you have beaten the regular story, revisit Weathertop (doing so after nightfall seems to work best). Use the rope on a hook and a board appears. Proceed to the end of the board and a trail of blue studs will appear that leads to a puddle. If you leap off the board, you'll hear the sound of an eagle and then land in the puddle, like an assassin leaping from a viewpoint and landing in a bale of hay or some brush in the Assassin's Creed games.