Note: Before you can use the ability to throw sand, you must first train at the Combat Trainer in Monteriggioni. Press to speak to the trainer and select - Special Moves - Unarmed Combat
The throw sand ability is free
to learn so remember to return to the Villa and learn at as soon as you can.
To throw sand target an enemy with
then press and hold
whilst you are unarmed (Ezio will crouch down and gather sand in a sweeping motion, once complete release Square to throw the sand.
This can be quite hard to get enough enemies hit with your sand so you'll want to get a big group of guards on you before trying, for example get one group on you and block (
) and move around to gather more. Any populated city will work though I found Venice to be good for this.You can throw sand as many times as you need without running out or hurting guards so if you don't get 4 on your first attempt just keep trying. If you need to you could throw down a smoke bomb to avoid the guards hitting you.
Heres a video by PowerPyx