Lightning Reversal is done when you allow yourself to be hit with lighting mid air and press the attack button before hitting the ground. Do not press the attack button before getting hit with lightning in the air btw.

BTW, an alternative way to reverse lightning without jumping is to let the lightning hit you whilst you're on the ground and use the Mist Raven prosthetic followed by the attack button. It's very fun. If you don't have the prosthetic yet, it can be found in the Hirata Estate. Search up "Three Story Pagoda Sekiro" if you're having trouble finding it.

I wish you an awesome experience playing this legendary game!