Someone else on here has a similar problem with really bad constipation, and rather than starting the day before, which is normal, s/he goes on a liquid diet and laxatives TWO full days before the procedure.
Although, I don't know what they would see on one that would indicate why you have constipation. If you have a kink in your guts, that's going to be in your small intestines, as will a hernia. And slow movement won't show up on a scope. Seems like a bit of a waste of a procedure to me (unless you're already middle aged, at which point they want to do one on everyone as a baseline to compare to later in life); a better bet would seem to be something like a barium contrast or pill camera, where they can test how fast your gut functions (in the case of the camera, they can also see in there as it goes down). If it gets to a spot and stops, that would be a pretty good indication that you have a blockage or a kink. If it goes through, only really slowly, then you're working too slow.