This is because the palm facing forward is imbued with their own defensive version of busoshoku haki known as Hardening depending on their will, depends on how strong it is. Whilst the fist tapping/hitting it is also using buso haki, it creates a forcive shockwave of pressure in the air that is turned into an animal and directed by the hand sign formed immediately after the attack. There are currently only two masters of this branch of Shishi-ō no mai.