Sivir gains 50 movement speed for 2 seconds when she hits an enemy champion with her abilities or basic attacks.
Standard AD pusher build. Building "Statikk" in early game gives you possibility to outfarm ("Statikk" works works well with "Ricochet") and incrases "range". Building "Blade of the Ruined King" makes you harder to kill in combat and gives u nice active to chase an enemy. Zephyr, Mallet Youmuu and Black Cleaver are optional items - all passives are adapted to your role in team, an enemy and yuor playing style.
Manamune with Sivir is necessary because her Q loses uses a lot of mana for a ADC.
@EmeraldStar: You don't need a manamune anymore with the new rework of Sivir because her mana costs are now lowered and if you can have fast enough reflexes to use your mana shield (e) you can regain mana very quickly.