Let me see if I got this right.

Throughout DGSD, before Duan Yu absorbed Jiumozhi's life energy, Duan Yu did not show any auto-protect capability. Is this correct? Thus, SWORD GOD was able to easily inflict a wound on Duan Yu's body. And Murong Fu was able to knock him down easily by slapping him in the face.

But after he received Jiumozhi's life energy, which almost doubled his internal energy base, he immediately demonstrated profound auto-protect function.

Here is a scene in which two such instances are illustrated:

一个箭步,便向段誉扑了过来,左手将书笺一把抢过,右手重重一拳,打向段誉胸口。 段誉正在思索虚竹信中所言是何意思,宗赞王子这一拳打到,全然没想到闪避,而以他武功,宗赞这一拳来得快如 电闪,便想避也避不了。砰的一声,正中前胸,段誉体内充盈鼓荡的内息立时生出反弹之力,但听得砰的一,跟着几下劈拍、呛啷、哎哟!宗赞王子直飞出数步之外,摔上一张茶几,几上茶壶,茶杯打 得片片粉碎。

这一次他学了乖,不敢再伸拳打段誉胸膛,抢到信笺,右足一抬,便踢中段誉的小腹,那脐下丹田正是炼气之士内 息的根源,内劲不听运转,反应立生,当真是有多快便多快,但听得呼的一声,又是劈拍、呛啷、哎哟一声响,宗赞王子倒飞出去,越过数十人的 头顶,撞翻了七八张茶几,这才摔倒。

Here, Duan Yu automatically repelled the Tibetan prince, who was the disciple of Jiumozhi and whom Jiumozhi remarked as having "pretty good" kung fu.

So, why do you think Duan Yu suddenly gained auto-protect function? Or, do you think he had it all along, just that SWORD GOD's simple stance was too great to be affected by Duan Yu's auto-protect?