So... this guide is simple and quick. I was going through some walkthroughs in youtube, since i can't do one myself, cause I don't know how to record my gameplay directly. I went to see how people play AC games, and I noticed no one knows how to drop onto somebody. I know there are people who actually know how, I am not the only one, but to most people, I talk about the drop action, and they don't know what I am talking about.  In single player, when you are on a rooftop and have a target or even a normal civilian on site, you can go and assassinate, pressing the weapon hand button. But sometimes you can't assassinate that specific target, or you just want to go down without dropping health, so most people sometimes end up jumping, or using the parachute (in revelations case) or try to drop carefully grabbing and letting go of ledges, kind of like Jet Li style. BUT,  for those who don't know,you actually can land on somebody, I will explain how to land on top of someone:

You are on the edge of the rooftop, and you have someone locked, or there is no need to have them locked, you just gotta press the empty hand button: 'O' = PS3 'B' = 360 (I don't know on PC) So what basically Altair or Ezio will do, is just jump onto a nearby person, or the person you have locked, or looking at, he will jump on him, like if performing an assassination kill, but without killing him. Now don't be scared, it might make the assassination sound, but it doesn't use the hidden blade.

I hope I could help, now I know a lot of people knows that tirck, but also a lot of people doesn't so I just wanted to let you guys know, and let it out there. I wish i had some footage of it, but like I said, I couldn't find nobody performing that trick on youtube, (I went through more than 50 vids, really, no joke) and I also don't know how to record my gameplay. I hope this could help to some of you, if you have a question, you can ask me in my inbox, or comment on my profile.