Rewritten in full by Faction Pack v1.2, so this wording is first-party. It has a defensive half, an offensive half, and a keywords clause.
Defensively. Each time an enemy unit is selected to shoot or fight and one or more TZEENTCH MUTANT INFANTRY or TZEENTCH MUTANT MOUNTED units from your army are selected as a target, each of those units can make a Warpmeld Sacrifice. If it does, until the end of the phase, each time an attack targets that unit, subtract 1 from the Wound roll. At the end of the phase, that unit suffers D3 mortal wounds.
Offensively. Each time such a unit is selected to shoot or fight, before selecting its targets, it can make a Warpmeld Sacrifice. If it does, until the end of the phase, each time a model in that unit makes an attack, add 1 to the Wound roll. At the end of the phase, that unit suffers D3 mortal wounds.
Keywords. TZAANGORS units have the BATTLELINE keyword, and while such a unit is not battle-shocked, add 1 to the Objective Control characteristic of TZAANGOR models in it.
Three things to understand before you play it.
The cost is per phase, per unit, and it is real. D3 mortal wounds averages two. A unit that sacrifices defensively in your opponent's Shooting phase, then offensively in its own Fight phase, has paid roughly four models' worth of wounds in a single round on top of whatever the enemy did to it. This detachment does not have a durability rule; it has a rule that trades durability for maths.
Both halves are optional and per-unit. You decide unit by unit and phase by phase. That is the skill: sacrificing with the cheap Tzaangor unit that is about to die anyway is almost free, while sacrificing with the unit holding your key objective can lose you the objective to your own rule.
A full wound-roll bracket in either direction is a lot. Minus 1 to wound against you, or plus 1 for you, is the single largest swing available for free in most games. Paid for in mortal wounds, it is priced honestly rather than generously, and the whole question is whether you have enough bodies to afford it.
And the Objective Control clause has a sting. Tzaangors get +1 OC only while not battle-shocked. So a battle-shocked Tzaangor unit loses both the bonus and, per the core rules, drops to OC 0 anyway. Battle-shock is this detachment's real enemy, and note that the enhancement that clears it belongs to the other mutant detachment, which you cannot take.