Sora wasn’t backing down, and while Riku knew he had the greater arm strength, Sora had the better foothold. Riku instead thrust Way to the Dawn upward, using the edge of the horn to try to catch the edge of Sora's Keyblade.

The delicate blade portion of Oathkeeper would fit nicely into that notch between wing and horn...

And if so, it was a matter of ripping it out of Sora's hands. Though Sora was excellent at resummoning his Keyblade, it would buy Riku a brief but precious opening.