It got established at least as far back as "Night Out" that if a female is zapped with Ellen's beam, she also gets the Female Variant 5 changes added to her, which generally results in longer hair, a larger bust, and a curvy-but-not-chubby figure.
However, the female elf here looks kind of dazed and the setup seems to imply that said elf was formerly-male and just got girlified. Note that she is sitting on the ground looking down at herself, which implies that what she sees isn't what she is used to.
The knockback/concussion part of the beam seems to happen to any target if Ellen uses full power. Anybody who is "normally" female also gets extra feminized, while anybody who isn't will become girlified.
Well, maybe they got unwanted present at Christmas. Maybe they noticed that they are more male elves that female ones and decide to fix that without convince them first. Maybe the work for Krampus or Père Fouettard and have to destroy Christmas. The possibilities are endless.