Subtle Indication of 'non-resisting' use HP for Knockback calculation?

[Agent]

GURPS Knockback blurb out of campaigns:

If the target has no ST score at all (like a wall), or is not resisting, use its HP instead.

The inanimate without ST like a wall part I get, but 'not resisting'?

I can only presume this isn't to mean a given biped with more HP than ST can say 'I'm not resisting' and gain a benefit. And also that it isn't to mean a given biped who cannot resist (unaware of attack perhaps, or by some trick unconscious and upright) can do this.

Even with realistic small variances between the two stats, its seemingly off, so...what does that latter mean? What has a ST score that is not resisting to indicate what this references?