In order to use the Sweep move, you must get trained by the Combat Trainer in Monteriggioni.
Go to Special Moves, Long Weapons. It will cost 10,000 Florins.
You will just need to get several guards to attack you, the more the better, then hold
, and release to sweep all around you.
It helps if you hold
longer so you spin your sweep in 360 degrees around you. As opposed to the shorter press, which just sweeps the guard in front of you.
Have a look at the video for some help.
Video is not mine, credit goes to MrBlingBlingNr1 on YouTube.
Credit goes to : Titan Eric from Trueachievements