The scene in question? It’s the flashback that explains Elsa Mars’ backstory, in the fourth episode — “Edward Mordrake: Part 2” — set during the 1932 German Weimer Republic.
The episode explains how Elsa struggled to survive in post-World War I Germany, and took employment at an exceptionally dark and violent brothel. Unfortunately, this attracted the unwanted attention of an even darker and grimmer element. Drugged and isolated, Elsa Mars became an unwilling participant in a brutal snuff film. The abductors tied Elsa to a bed, and then proceeded to film the action of slowly cutting her legs off with a chainsaw. In a voiceover, Elsa comments that after the deed was done, the psychopaths left her to bleed out and die. She adds that had it not been for a “soldier boy in love” she would not have survived the ordeal.
In the penultimate episode, “Showstoppers,” it is revealed that the mastermind of Elsa’s crippling is none other then the monstrous Doctor Arthur Arden (James Cromwell), a former Nazi officer whose real name is Hans Gruber. He was previously one of the main characters in “American Horror Story: Asylum.” And while “American Horror Story” has featured plenty of dark or grotesque scenes in the past, this particular flashback elicited a strong, disturbed reaction from the internet.