There is something a bit odd with the 4th ability in STA. At first, I thought it was an additional chance to riposte (that adds to your base chance). Ripostes only happen if you have already successfully parried, so no avoidance gained. I no longer believe this is the case. I believe it acts like a proc and that it's checked before parry. Since it's given as a %, I also believe it's uncontested. If it's true, it puts a buckler very close to a tower shield.