I think the phrase comes from " Multiplication Mind Jigglers", a math exercise to practice multiplication. A Jiggler is the noun for jiggle, which is short movements up and down or to the sides, and i think it refers to how the children who do the exercise have to "hop" around the page to find the right answer.
Since 3 is evenly divided by 9, we can multiply just one term to get a common denominator. Multiply 2 by 3, and get 6, then we multiply 3 by 3 and get 9.
