The exchange of blood is important in many Blood Angels rituals, which are presided over by the Sanguinary High Priests. In battle, the Priests use an Exsanguinator to transfer their blood to the injured, imbuing their battle-brothers with the endurance of Sanguinius. A model with an Exsanguinator allows the squad he is in to ignore the first failed armour save each turn. In addition, the Sanguinary Priest can try to save more of his brethren with his own life fluid and attempt to negate second and subsequent failed armour saves. To do so, roll a D6. If the score is equal to or less than the Sanguinary Priest's Toughness, the failed armour save is ignored. If it is higher than the Priest's Toughness, the model which failed its armour save and the Priest both suffer a wound with no armour save allowed. An Exsanguinator cannot negate hits from a weapon that inflicts instant death (ie, has a Strength of double or more the target's Toughness) and it cannot be used if the Priest is in base contact with an enemy model.
Exsanguinator
Source: Codex: Blood Angels, p. 17