One clause, applied broadly: each time an attack targets a HAEMONCULUS COVENS unit from your army, if the Strength characteristic of that attack is greater than the Toughness characteristic of your unit, subtract 1 from the Wound roll.
That is the same shape as the 1-DP Tools of Torment rule, but where that one covers Cronos and Talos only, this one covers every Haemonculus Covens unit: your Wracks, your Haemonculi and your Pain Engines alike. Paying an extra Detachment Point buys you scope, not strength.
The direction is worth internalising. It blunts high-Strength attacks, so lascannons, melta, power fists and monstrous claws all get worse. It does nothing against massed low-Strength shooting, because that Strength is not greater than your Toughness. The counter to a Coven list is volume, and knowing that tells you which enemy units to kill first.
On a Toughness 5 Wracks unit that means anything Strength 6 or higher is wounding one step worse than the dice say, which is most of the anti-infantry heavy weapons in the game.