Person A has position X.
Person B disregards certain key points of X and instead presents the superficially similar position Y. Thus, Y is a resulting distorted version of X and can be set up in several ways, including:
Presenting a misrepresentation of the opponent's position.
Quoting an opponent's words out of context — i.e. choosing quotations that misrepresent the opponent's actual intentions.
Presenting someone who defends a position poorly as the defender, then refuting that person's arguments — thus giving the appearance that every upholder of that position (and thus the position itself) has been defeated.
Inventing a fictitious persona with actions or beliefs which are then criticized, implying that the person represents a group of whom the speaker is critical.
Oversimplifying an opponent's argument, then attacking this oversimplified version.
Person B attacks position Y, concluding that X is false/incorrect/flawed.
This sort of "reasoning" is fallacious, because attacking a distorted version of a position fails to constitute an attack on the actual position.