A technique deployed by villains in order to disable the hero, the Darth Vader Gambit involves imparting a piece of traumatic exposition at a crucial moment in battle, immediately precipitating an emotional crisis that will cause the hero's weapon to clatter to the floor and the hero themself to fall to their knees, clutching their face. Often this is accompanied by violent trembling and repeated utterances of the phrase, "No... it's not true... it CAN'T BE true..!" At this point the villain may choose to respond with, "Look deep within yourself... you know it to be true." at which point the hero will reply, "NOOOOOOOOOO!"or some other variant on the template.
Watch out for phrases such as, "Have your friends not told you what they are truly planning?", "Do you know who really killed your brother/sister/parents/teacher/wife?", and the evergreen "You and I are alike..."
The only known countermeasure to the Darth Vader Gambit is the Tuco Defence. As far as I know, however, no anime hero has ever tried this.
Note that it is even more infuriating for dedicated Tuco acolytes when the Darth Vader Gambit is used in a video game, since the computer will wrestle control of your character from you and force you to sit, helpless, spitting obscenities at the screen, as your character flops about on the ground like a recently caught salmon.