Charge-up Skill: A properly trained Assassin can focus her mind to draw upon the ambient energies surrounding her. Using this skill, she can drain her adversary of life and spiritual essence.
Cobra Strike is a charge up that allows the assassin to lifesteal with the one charge, lifesteal and manasteal with two charges, and lifesteal and manasteal moreso with three charges. As such, if an assassin has a good physical damage and attack rating, Cobra Strike can sustain life and mana fairly dependably. However, since lifesteal and manasteal are reduced by 1/2 in nightmare and 2/3 in hellmode, and since monster damage resistances go up in hell (and in nm as well for some cases), the result is that Cobra Strike works best in normal.
Lifesteal and manasteal also does not work at all on some mobs for any difficulty (such as skeletons, flying scimitars, gargoyle traps, swarms, lightning spires, etc). Using Cobra Strike, the assassin will not be able to steal life or mana from these kind of mobs.
Many melee (PvM) assassin builds include Cobra Strike as an important skill for the build, but only allocate one point to it. At base lvl 1 (with additional skills from gear) it will provide most of its utility.