Your training and practice with shields has taught you how to best utilise your shield to its fullest potential, allowing you to learn the following:
Keen Blocking The bonus to AC you recieve from using a shield increases to +4 when you can see the creature attacking you.
Shield Bash When engaged in combat, you may use your offhand attack to shove the creature you are fighting away from you. The creature must succeed on a constitution saving throw against a DC of 8 + your proficiency bonus + your strength modifier. On a failed save, the creature is pushed back 5 feet, and then knocked prone. On a successful save, the creature remains where it is, and can take an attack of opportunity as you lower your guard. When defending this opportunity attack, you do not get any shield bonus to your AC, as your guard is lowered.
Shield Barge When you take movement during your turn to engage a creature, you can take a bonus action immediately after engaging to attempt to ram it with your shield. You must make an attack roll, with a modifier of your proficiency bonus + your strength modifier. On a hit, the creature takes 1d4 bludgeoning damage, and must succeed on a Constitution saving throw against your attack roll. On a failed save, the creature is knocked prone. On a successful save, the creature remains upright.