With DUI, you need only climb into your vehicle while under the influence of alchohol or drugs and attempt to drive it home.
With MOUI you must disable a number of safety systems designed to prevent idiots like you from manually operating their vehicles while inebriated, overtired, wasted, decaffeinated, angry, emotionally distraught, or suffering from hormonal disorders like PMS or testosterone poisoning (the latter having been positively identified as a leading cause of stupidity among males between the ages of puberty and death). After disabling the safety systems (which task almost certainly requires ice-cold sobriety), you must decide to switch the vehicle to a manual mode of operation. In some cases, this requires installing a manual mode of operation.
After this demonstration of superior technical skills and poor judgement, you must then deliberately impair your defective judgement further by quaffing, inhaling, injecting, or otherwise consuming something mildly toxic (or, in some precedent-setting cases, breaking up with your girlfriend.)
At this point you might argue that you are no longer responsible for your actions, which actions include climbing into your vehicle and attempting to pilot it home. Technically, this is true. The prosecution will counter by arguing that it isn't Manual Operation Under the Influence that you are going to be humanely (if rather publicly) executed for. It's the fact that you deliberately made such an irresponsible act POSSIBLE by putzing around with your ride. You know those signs that say "don't putz around with this system -- serious injury or death could result?" Well, they were talking about YOUR death, and it is now resulting.