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Activation: When you make a Martial Arts [Melee or Unarmed] check, if you have a second readied weapon you did not use for the attack, you may spend [opportunity] in the following way:

• Given the wording, am I allowed to make an 'off hand' strike after a Martial Arts [Melee] check which was not an attack - for example a Centre or Guard action? If not, it should probably specify 'An Attack Action'.
• This action also allows a pseudo-strike to be landed for two opportunities, regardless of what the TN of a strike action against the target would normally be (much like the Charge action prior to the beta update 1), which seems wrong.

Agreed for the 1st point.

For the 2nd one maybe it should add the condition that the first attack hits?

4e spinning blades used to let you do the extra attack maneuver with your off hand for fewer raises than usual, so that would be consistent with the idea.