Show-Off Achievement in Assassin's Creed: Revelations
Parachute onto a zipline.
How to unlock Show-Off
You may think this needs to be done from high up, but it doesn't. This means whenever you find a zipline you can do it. You should have parachutes unless you have used them all. If this is a case, go to a tailor and purchase them. Find a zipline and climb up it (not to the very top though, just the part the line is attached to). Now jump, hold and then let go and hold . If you do this quickly enough, you will parachute and then drop onto the zipline.
Here's a video for further assistance: