It is passive skill of temple knight and shilen knight learned at lvl 60 or 62. It says provides shield defense from all directions when shield is equipped.
There is something implemented in game about the shields and defend rate. You can test it by attacking a low lvl mob let say with unit that have shield and better to have high level shield mastery. You can see after take some hits from mob how some hits are defended by shield the mob that. Now turn your back to the mob and let it continue to hit you. You will see that shield defend rate is 0 at this moment. If you turn again towards the mob success shield defend rate will start to appear again. I have tested that.
And here comes this Aegis passive skill. It should allow your char to block with shield no matter from where attacks are incoming. But it does not. It is the same as described above. So there should be a check of shield defend rate to see from where attacks are coming and to apply shield defend only if attack come from front of the char. Aegis should remove or disable that check. Also Paladin and dark avenger (not sure for second one) have similar skill but it is toggle skill. It description says If I remember well that provides shield defense from any directions but lowers shield defend power. Since they are not popular chars this reduction in def power of shield maybe better is not to be added.