Source: Starfinder Core Rulebook p.63
As a reaction, when an enemy makes a ranged attack against an ally within 60 feet of you, you can warn that ally of the danger. You must spend your reaction when the enemy declares the attack but before it makes the attack roll. Your ally can spend a reaction to gain a +4 to AC against the triggering attack. Once the triggering attack is resolved, the ally falls prone.
At 8th level, you can spend 1 Resolve Point to prevent your ally from falling prone after the attack.