I have always just assumed that only a small amount went into the waste tank, just enough to make sure that all the lines actually were filled. Since an initial prime takes about 1/4 of each cart, most of which (I assume) ends up in the lines, and the second prime just pushes all that ink into the waste tanks, it's an expensive mistake if the first prime was almost complete before it aborted. I actually made the mistake I'm warning against, live and learn, as they say.....