What's GURPS 101? Just basic explanations of the GURPS rules as written, in an attempt to make a specific topic clear to someone just getting used to the game. Or, as a refresher for more experienced players and GMs who may have changed or forgotten some of those rules. Any house rules and optional rules on the subject will be in a followup post.
Last week I wrote a GURPS 101 post about Knockdown and Stunning and consciousness rolling when wounded. It was rightly pointed out, though, that stunning and knockdown and stunning are not always the same. Nothing in the previous post said they were, but it's an easy enough mistake to make to think that "stunning" always means "use the Knockdown and Stunning rules." Therefore, here is a look at Stunning.
Stunning is defined in GURPS Basic Set: Campaigns (p. B420), right after Knockdown and Stunning. They can be easily confused with each other or conflates, but they aren't the same.
Stunning is a condition of restricted actions and reduced defenses. How does it occur? "A failed knockdown roll can cause “stun,” as can certain critical hit results and some afflictions." That isn't an exhaustive list - some spells can cause stunning, for example, without being an affliction per se. The sources are physical, or supernatural effects which cause a physical stunning effect.
Note that first phrase especially - a failed knockdown roll causes stunning (putting aside any immunities to stunning.) Get knocked down, get stunned or (if you roll badly enough) fall unconscious. Get stunned, and you do not automatically fall down.
Stunning recovery is a HT roll. There are generally no bonuses or penalties from advantages or disadvantages except for those that specifically add to all HT rolls, such as Fit or Very Fit. Some spells, and some powers (such as a stunning surge from a lightning-based attack) can inflict penalties.
Mental Stun works just like stunning. It represents confusion or mental shock or surprise, and can come about from Surprise Attacks and Initiative (p. B393), some spells or afflictions, Fright Check rolls, and other non-physical sources. Unlike normal stunning, your roll is IQ based, and it's not uncommon to have penalties or bonuses (such as the cumulative bonus to recover from surprise - see p. B393) to the roll. The effects of mental stunning are exactly the same as normal stunning.
Hopefully that clears up any confusion about stunning and knockdown and how they interact. Again, in short, you get stunned (or knocked out!) from a Knockdown and Stunning roll, but you can get Stunned without getting knocked down!