Unmoving (Ex)Source Armor Master's Handbook pg. 15
Select one combat maneuver (except sunder). Whether using his armor as leverage against a grappling foe or using its weight to help him stop a bull rush, a fighter can use his armor to protect him from the chosen combat maneuver. While wearing armor or wielding a shield, the fighter gains a bonus to his CMD against the chosen combat maneuver. The bonus is +1 if he’s wearing light armor, +2 if he’s wearing medium armor, and +3 if he’s wearing heavy armor. This bonus increases by 1 at 7th level and every 4 fighter levels thereafter, to a maximum at 19th level of +5 for light armor, +6 for medium armor, and +7 for heavy armor. Mithral armor provides a bonus 1 lower than normal for armor of its type.