In Assassin’s Creed 3, you can tackle people from above to surprise them and knock them to the ground. There is a mission later in the game where you are required to do this as well.
To tackle from above, you just need to jump down onto someone from above, then press and hold B on the Xbox 360, or Circle on the PS3 right before you land on them.
If anyone knows the button config on PC or Wii U, please leave me a comment with them.