This works just like the zipline assassinations in Assassins Creed: Revelations. It is easy to acquire in the heavily ziplined area just south of the Assassin's Bureau in Sinai. Some of the drops are very far and so the prompt will only be available briefly. It is the
key and you can simply mash it as you slide to assassinate when you pass over a viable target. You can even get some people who don't quite seem to be directly below you by doing this.
You will clear the quarry area with all the ziplines out during one your assassination missions, but it is difficult to do multiple zipline kills without being detected when all the enemies are up. I had the best luck with the one zipline that goes east to west across the northern part of the area, as opposed to all the ziplines that cross the canyon from one side to another (where you're often detected before you make it all the way across).