Rallying Cry gives every party and raid member within 40 yards a temporary 10% boost to maximum health for 10 seconds. It maintains health percentage the whole time, current health scales up on cast and back down on expiry, so it never heals anyone and never drops anyone below where they were.

In practice it is purely a window of extra effective HP: for 10 seconds the raid can survive a hit that would otherwise be lethal, but the spike has to land inside that window to matter.

Treat Rallying Cry like a raid damage reduction: assign it to lethal damage bursts the raid needs help surviving. Its edge over ground-based cooldowns like Anti-Magic Zone, Darkness, and Power Word: Barrier is that the raid can stay spread out and still benefit.