The New Erotic Ninjutsu Completed!!

is a manga-omake included with Kai no Sho.

Summary
This omake is set during the two-and-a-half years that Naruto spends training with Jiraiya between Part I and Part II. After a day of training, Naruto is eager to cook a fish that they caught. Jiraiya, meanwhile, has writer's block for his next Icha Icha novel, and is unwilling to cook Naruto's fish. Naruto uses the New Sexy Technique to convince Jiraiya, and Jiraiya happily complies. As Naruto celebrates, Jiraiya decides to use Naruto's Sexy Technique as inspiration. He suggests Naruto wear a thong as part of his transformation. Confusing a thong for a mawashi, Naruto transforms into a sumo wrestler. When Jiraiya becomes angry with him, he uses the form to knock Jiraiya out.

Trivia

 * This omake (and its anime adaption in Naruto: Shippūden episode 188) marks the only time Jiraiya has ever used the Fire Release: Dragon Fire Technique.