I was told the point is to drink liquids/shakes. The liquid process is for the liver to shrink, so that they can perform the surgery more effectively. Additionally, to train you to survive off of liquids post-op. I was told that even though you are on a liquid diet you should still try to drink as close to 1,200 calories as possible. so 3 shakes at 160=480cal.
And last thing they also told me that I can have some limited things; like vegetables (once a day), yogurt, cottage cheese. oatmeal, everything has to be plain. no butter, seasonings, salad dressing, sugar, etc. I got a lot of info from the internet also. Search "shrink liver"
Lastly, NO soda or caffeine.