One Hand and Shield Expertise is a skill that can be used by human characters. Once trained in this skill, it is always in effect.
Skill Text: "Those who engage in battle with sword and shield are sometimes scoffed by others on the premise that they are archaic and unimaginative. But they never take such criticisms to heart because they know that their strategy, classical rather than standard though it may be, is still one of the best setups a warrior may use: it perfectly combines offense and defense and, like no other, offers excellent chances to emerge from each battle alive and well."
The armor bonus applies to your shield's melee, ranged and magical armor bonuses but only adds whole numbers. (i.e The first level may add 5% armor to a 10 armor shield but that is a total of 10.5 armor which would not increase the damage mitigation. At level 3 however, the additional 11% would turn 10 armor into 11.1 armor which would increase damage mitigation.)