Re: Ushiro ate-atemi waza?
None of the Tomiki atemi waza are percussive but are best viewed as using pressure to manipulate uke's body. My personal emphasis is spine but that may or may not be the best way of looking at it.
Ushio-ate actually involves a downward pressure with the small palm coupled with the tori moving backward to effectively collapse uke.
God I love that technique which is why I have it as my gif.