Pg. 194 of DW says that Iron Arm is a half action or reaction. While this power is in effect the Librarian gets an additional Reaction each round to parry. If this power is activated as a Reaction, does that mean he only gets one parry that first time, but two parries everyone other time (as long as it is sustained?)

Repentinus said:

If my understanding is correct then it should be as follows:

Activating Iron Arm as a Reaction:
(From P194 Core Rule Book) Each character is allowed one reaction per turn that means you can either choose to Parry/Dodge. If you activate Iron Arm as a reaction, then you forgo the first opportunity to use your reaction to dodge/ parry.

Essentially its like trading a reaction for dodge or parry for a parry + WS bonus = 3x current PR (taking into account power level manifestation of fettered, unfettered, or push).

Activating Iron Arm as a Half Action:
If you activate this as a Half Action, you then have one reaction (for dodge/parry) + one from Iron Arm (which can only be used to parry only).

Sustaining Iron Arm after manifestation (on subsequent rounds):
You spend half an action to sustain the already manifested power of Iron Arm. Benefits are the same as Activating Iron Arm as a Half Action.

Also, if you have attacked an enemy with a physical attack and manifested your psychic energy as a reaction (to boost damage) then you can't manifest Iron Arm as a reaction.