
Fast forward to after the Outsiders have "converted", Kiara is dangling off the edge of the cliffs, with a raging river right below her ready to sweep her away to her death. When Simba went to save Kiara, the river "stops" afterwards and Kiara and Simba just walk up... somehow. Now how about the river is still going on and Simba has trouble trying to save Kiara. So when Simba grabs Kiara's paw, Simba is about to fall or trip on the cliff. And then, Kovu helps him getting up.
If you have troubling trying to comprehend this, then, I try to break it down one more time. Simba successfully rescues his relative, Kiara, would have been a mirror to Simba unsuccessfully saving his relative, Mufasa in his nightmare. Except, instead of Kovu throwing Simba off the cliff, he helps him hang on the cliff so Simba can save his relative from sudden death.

Personally, I think this was a missed opportunity to add depth into the story, and Kovu should have helped saved Simba save Kiara since he told Zira that he would never let her hurt Kiara.