Mercy kill and air assassination
When you kill someone using your sword or blade they usually roll around on the floor making a lot of noise and attracting attention. If they are still alive and moving on the floor select your secret blade, press up, and go upto the body and press X. You will then finish them off.
You can assassinate targets/guards/civilians from above. Stand on the edge of a ledge not too far from the ground. Not too high or he won't do it. Lock onto your target and make sure that your secret blade is selected. Then hold RT, lean forward and press X and he will automatically jump down and assassinate. Note that this will be a loud assassination and will attract that attention of the guards.
More tricks, tips and cheats for this game are right here - Assassins Creed cheats
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