Make sure you have purchased the throwing knives special attack. Make sure you have at least 1 knife in your inventory and climb to the top of any tall building.

Equip the throwing knives and run/walk/sprint (any of them will work) at the edge of the building and hold X.

Make sure you press X after you have started moving or it wont work. Keep running until you reach the edge of the building and Ezio will walk off the edge of the building and keep walking over the air as if the building never ended.

Ezio will keep going like this until you release X, at which point he will fall out of the air and to the ground. No other buttons will work after he is in the air, and you cannot stop running or turn until you fall.

You can run all the way out of the map if you want, and reach unclimbable locations like castle walls and such!