Episode 21 can be read here.
On there way to Atmos City, Ash and Pikachu meet a trainer when he literally falls out of the sky. The trainer then challenges Ash to special type of battle, one that is fought while skydiving. Ash eagerly accepts the challenge and learns the intricacies of a skydiving battle.
Also, the Pokédex and Tenno League pages have been updated accordingly.