I’m not really sure why you’re expecting it to be different to ratchets on other kinds of goals, or what level of detail you’re looking for, so it’s hard to get at what you’re asking, is the problem! So apologies if this misses the mark.
All ratchets put a road entry in the matrix which keeps the graph-to-date (since the last road entry) at the same rate, and then add a new entry which moves your road to whatever value makes the graph due again.
With your whittle down graph, it’s just the same. If your rate is -1/day, you’re at 8 total, and you want the graph to be due today so you have to enter 7 total, you set the ratchet to 0, and the line will drop to 7, like this:
…Which makes a road matrix like this:
The sole difference from how it works with a do-more ratchet is that the “value” column has decreasing numbers rather than increasing numbers, and the rates in the rate column are negative.
In practice, you don’t need to worry about the nitty-gritty. You should just need to ratchet to whatever date you want, and the graph will do it, so I don’t think we have documented anywhere user-visible exactly how it works from a technical point of view – just what it does, i.e. make the goal due on the day you ask us to make it due. I can totally be convinced to pitch that as something to add to the Help Docs, if you can explain why you feel it’s necessary to know how we ratchet? Or what you feel is missing from the current documentation? It’s really helpful to know what folks are looking for!