One of the challenges for girls who are learning on the uneven bars is mastering the kip. Kips are important moves that are necessary to do well on both bars. Doing kips requires strength, skill, and coordination. This must be learned and practiced. In this video a gymnastics teacher explains some of the basic ways that a student can learn and practice kips.
As you can see, it takes a lot of strength conditioning and building flexibility in order to become good at doing kips.