Must be played (as well as other "as the action is announced" action modifiers) before action modifiers that are not explicitly played "as the action is announced" (e.g., Conditioning).
101122
Loose Cannon
Requires an Anarch. Only usable as the action is announced. If this action is blocked, apply the following effect before block resolution:
Lock a younger non-blocking vampire controlled by the blocking minion's controller.
Move 2 blood from a non-blocking vampire controlled by the blocking minion's controller to this vampire.
Damage from this vampire's hand strikes is aggravated during the resulting combat.
Rulings
Replace the action card (if any) before playing it.
When played, a multi-Disciplines card counts as requiring the Discipline(s) being used. In the hand, library, or ash heap, the card is considered to require any and/or all Disciplines listed on it.
If blocked, the effect must be applied, ordered by the acting Methuselah, before any other card or effect can be used (eg. Crypt's Sons, Change of Target).