Thanks to dvdshrink for developing the feature
Thanks to Sbofen at doom9.org for finding it.
Thanks to ddlooping at digital-digest.com for making it clear to me

When you first open a DVD, you’ll notice that the “Automatic” and “Ratio” figures are the same but only the “Automatic” radio button is highlighted. What we’re going to do is apply different ratios to different parts of the movie.



If you click the “Ratio” radio button, a small meter bar appears and the cursor changes to a ‘grasping’ hand allowing you to slide it back and forth. To achieve finer control, maximise the window.



In and of itself, it appears useless, unless you want to pick a compression level and then aim for it, deselecting this and that, while watching the green bar all the time. What you can do is select different compression levels for different parts of the video stream but it’s not immediately obvious how to do this. If want to keep as much of the DVD as possible, or perhaps provide high quality for selected parts, then some tweaking of individual elements of the DVD [title] is required. For example, maybe providing high compression to the opening and closing credits, and maybe a silly, lousy scene in the middle. Maybe, providing for no compression for one scene or chapter and adjusting other elements of the movie accordingly. Click onto “Automatic” compression and open a Re-author window. Drag the main movie over for 56% compression (in my case). Click Start/End and find the opening credits. I found 33 seconds or 9MBs worth.



Click “OK” and you can see that my ‘new’ title shows up. Don’t panic, the main movie has not disappeared! Now I can play with the ratio, giving me 47% at 3MB rather than leaving it out or “Still-Picturing” it.



Now I’ll Still-Picture the end-credits, in my case, from just before Chapter 52 all the way to the end of Chapter 54. Click Start/End and find the end-credits (I get 229MB).



Click “OK” and then ‘Still-Picture’ them with the appropriate radio button (37MB @ 20%).



Then I’m going to apply no compression to a few Chapters. Scenes with smoke, fog or lighting effects, or close-up work do poorly with high levels of compression. If you’re serious, it may take an hour or two to work out where high or low levels of compression should be applied. Click Start/End and choose your favourite scenes – I’m choosing 5 Chapters (10-14) then selecting “No Compression”. It’s now locked in and remembered like the previous changes we’ve made. The start frame means "start of" and the end frame means "end of"... so if you set both start and end to chapter 10, that will cover the whole chapter. If you set start frame to chapter 10, and end frame to chapter 14, then you will cover five chapters.



Click “OK” then “No Compression” to lock it in.



Now I’m going to scrub Chapter 49 down to around 50%. Click Start/End, select the Chapter in both drop-down boxes, click OK and ‘ratio-down’ to 51%.





For the last step we use Start/End one more time but we select the whole Title.



Click “OK”. Check it out! I’ve got different compression at different points of the movie. Yep, I know the Megabytes look to make no sense (they actually do) but it’s done exactly what I wanted.



Now just hit that Backup! Button

Good luck. Got problems? Then post here