The instant you're injured to 0 HP or less: No matter
what maneuver you selected before being injured, you must roll vs. HT to avoid unconsciousness. Do Nothing – as a choice on that turn or as a consequence of something else – is beside the point.
Turns after you're injured to 0 HP or less: If you select
any maneuver but Do Nothing, you must roll vs. HT to avoid unconsciousness before you take action. If you select Do Nothing – as a choice on that turn or as a consequence of something else – you do not have to roll but of course have no action to take.
That is, my clarification was of the two offending sentences. The Do Nothing exception isn't in either of them, and works as it always did: It lets you avoid a HT roll. But that's neither here nor there in what I was trying to clarify. The whole rule would be:
0 HP or less – You are in immediate danger of collapse. In addition to the above effects, make an immediate HT roll, at -1 per full multiple of HP below zero. Failure means you fall unconscious (or simply stop working, if you weren’t truly alive or conscious in the first place); see Recovering from Unconsciousness (p. 423). Success means you can act normally, but must roll again at the start of every turn to continue functioning. Exception: If you choose Do Nothing on your turn, and do not attempt any defense rolls, you can remain conscious without rolling. Roll only on turns during which you attempt a defense roll or choose a maneuver other than Do Nothing.