[Steve Plambeck]

I don't believe Defend should be construed as an instant of single action. It is an announcement of a form of tactical behavior the defender will engage in for the course of the full turn. It'll be looking out for attacking swings from any direction and attempting to duck or dodge as many as possible (and to the exclusion of making an attack on anyone itself).

Recall that if a figure chooses Defend, each and every figure that attacks it that turn, each in their order of adjDX, will have to roll 4d6 to hit. That could be six attackers if fully encircled (plus more people jabbing, throwing, and shooting from further away). The defender isn't choosing Defend each time another attacker takes their swing (as if it had multiple actions in one turn). Defend is a once-and-done choice that establishes a condition for the remainder of the combat turn.

Once you pick Defend, you can't go back on that choice for the rest of the turn. You just don't have to say you're picking it until you have to. It doesn't mean you are acting out of turn.

Think of things like this as equivalent to "waiting". You can't say to someone "Now hold on, you can't say you're waiting yet, it isn't you're turn to wait! You'll have to wait before you can wait!"