Hopinso??? (Why does standing up ruin the flush?)

Aren't you going to answer this nice person, hopinso? :)

Maybe nobody replied right away because your story seems to answer itself. You seem to already know that the Clark procedure (and others?) says to lie down right away, or else you might not get any stones out.

But does anyone know why that is the case? This is a mysterious aspect of the flush to me. Why should lying down matter?

Also, don't forget that the first flush is often much less productive than the second. (If it has been five years, then in the practical sense it was your first flush again.)