It makes sense to me, I experience that sometimes.

When we are triggered and shifting seats to use your analogy (so still present to a degree) grounding techniques can help to get more grounded and in control. Our therapist is good at helping with this. Out of therapy sometimes we can be aware enough and have sufficient presence of mind to do it and other times we can't.

When another alter is fully triggered out for me I am not in control at all or even aware to do grounding exercise. Sometimes a young part of us is good at jumping in and doing grounding things when another part is triggered, that must be her job I suppose.

To answer your question from a trauma perspective rather than a "you" perspective (because I don't have perspective on that) I guess grounding techniques help to calm the hyper-aroused nervous system which could result in either a full reset back to 0 (fully you) or merely a calmer hyperaroused state (still triggered and co-presence of alters). I don't know if that makes sense.