There is ALWAYS shared weight in AT, that's what makes the purely physical communication between two bodies possible, The weight is shared ALWAYS by surrendering of part of one's balance at the level of the torso's center and NEVER at the level of the whole body's center ( well, except when the follower is led into a lean, of course). How much weight is shared and in exactly what geometric shape of course depends on the style and on the embrace (open, V-shaped, "apilado", etc...)
So what happens if we imagine that one's partner is suddenly removed? The other partner should feel disbalanced in some degree or another (depending on the the embrace, the dynamic of that particular moment of the dance) because he/she has surrendered part of his balance, but he/she should never feel close to falling, because he/she keeps the center of whole body for his/herself.
I don't want to make this too long so I'll leave room for objections now

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But in my next post I'll try to explain why I think that this is not a purely academic point or something like that, and why this reiterated common "truth" has had quite bad impact on the AT community as a whole...