Arioch is correct.
When using Adrenal or Agile defense, it requires 10% of your activity. Which means that you can only attack, normally, with 90% of your activity maximum, thus giving a -10 to OB.
However, there are certain specific maneuvers that require 100% activity to perform.
For example, the Defensive Block, you take a -10 modifier to your init for the round, and for every -1 to OB used in the maneuver, you gain +2 to DB.
However, if you have Adrenal or Agile Defense as part of your style, then you have a choice. Drop the Agile Defense while performing the Defensive Block, OR take a -20 modifier to your OB, prior to moving OB to DB through the Defensive Block. (which still requires 100% activity).
Therefore, if you had an OB of 75 with a style that includes Adrenal/Agile Defense, your actual OB for a basic strike would be 65 with the Defense being used. Your amount of available OB for a Defensive Block would be either 75 (and no Agile/Adrenal Defense bonuses to DB) or 55 (-20 from using Adrenal/Agile Defense with a maneuver that requires 100% activity).
Is that making more sense now?