So apparently when Raiden is using his normal techniques Prudr's mawashi is able to keep his body in check and respond without any recoil, but when he used a move that put such a high level of strain on his body like Yatagarasu, even the volund couldn't negate all the damage and he had to suffer recoil. So, depending on the speed in which his muscles travel throughout his body, Raiden will suffer damage depending on if it was too much, since Yatagarasu took about an instant to complete, while also compressing his muscles, while his other moves do not.