With Knockback being optional now, when, specifically, is the decision to use it made? When attacking? When figuring up damage taken? This especially matters with variable damage sources like Blades/Claws/Fangs.

Thanks in advance!

I think is just before clicking damage

Step 6 of the Attack Sequence, so, after damage is clicked, you can check to see what powers/stats they have and determine if you want them to be knocked back. However, it's all or none, so you can't selectively knockback only certain characters.

2018 PAC:

KNOCKBACK
When one or more opposing characters takes damage from this character's attack, you may choose to knock back all hit characters an amount equal to their damage clicked

(emphasis mine)

You choose whether or not to knock back a character when they take damage.

[Misread the question. Ignore this post.]