Every successful critical hit with the Hirudo will return 5% of damage dealt to the enemy as health to the player.
Each successful critical hit also increases the player's max health by 5%, referred to as "Invigorated" by the HUD on the upper right corner of the screen. The health bonus lasts for 15 seconds, and if the player lands another critical hit during that period the buff duration is refreshed along with the heath bonus stacking for increased health. The Invigorated status can stack up to 5 times, for a maximum of 25% additional health bonus.
The Hirudo's max health bonus is scaled off of the Warframe's health at rank 0 in the same way that bonus health from the Vitality mod is.
Both the Hirudo's special abilities also work when using quick melee.
Critical strike life steal bypasses sentient immunity.
Since it only requires Critical Hit to activate both abilities and it doesn't require to switch to melee and energy, mod with True Steel, Blood Rush or Maiming Strike to increase Critical Chance and any damage mods to increase lifesteal.
Berserker is useful in heightening one's attack speed in order to initiate more critical hits.
A fully ranked Blood Rush along with increased melee attack speed will easily allow one to heal faster than most enemies can deal damage.
Using a warframe with higher armor values can help one stay alive while finding a target for the Hirudo's life steal.
Using this weapon with either stance equipped as Inaros can make charge attacks quite effective, as landing a charge attack on an enemy who remains alive after it connects will instantly transition into a finisher, which will activate Inaros' passive to gain health on finishers.