The colonic irrigation experience depends a lot on you and the therapist and the kind of system used.
If you are constipated, an open system is probably the better choice. You just lie back with the small tube in your rectum and relax and let the water flow in and out. There is no need to try and take and hold a fill, as with an enema. After a while, the water will break up the hard poo in you and you will start to have a movement. After some time, your bowels will move pretty much effortlessly. Because of the gentleness an open system is good if you are constipated or for a withholder.
To get a really full cleanout on an open system after you have started to poo, clench your anus and take a big fill of water. You can take as much water as you want, completely without fear of dribbling or losing control and making a mess. The sessions are very relaxing and after a few fills and releases, it is easy to take 2 or 3 quarts of water or more. It is quite possible to fill your entire colon, then release with a mighty gush. The effectiveness of the session depends on how much you take during the fill.
Several more things: With an open system, tummy massage helps to move the water up higher and later down and out, but many open system therapists leave you alone, so you have to do your own massage. There are benefits to privacy, of course. Also to get a really full cleanout, you may need more than the hour. Ask about extra time. Since your colon absorbs water during the session, you may need to pee. This is easy on an open system, although you may want to turn off the water inflow while you go. Finally, the hand sprayer on an open system is very good for cleanup of the whole area and can feel pretty good.
A closed system can be problematic, if you are constipated. I've found that a preliminary enema is a very good idea. It will help you evacuate hard poo and prevent clogging on the speculum. A speculum clogged with poo won't allow anything to pass and you may have a lot of water in your bowel. Like as not, you will empty everything on the table when the clogged speculum is removed. You'll never make it to the toilet.
The preparatory enema should not be too irritating. Nothing exotic that will stimulate your bowel making the irrigation session more difficult. My usual choice is warm water, with perhaps a bit of enema soap. The goal is to poo both hard and soft on the toilet before starting the colonic. Stay on the toilet until you have completely emptied.
I would urge that only disposable speculums be used and the machine FDA approved. There are a lot of nasty microcritters around these days. With the preliminary enema, you can use either the one or two piece speculums without worrying about a clog.
During the session, a good therapist will massage your tummy and can thus break up any remaining poo and urge the water higher. In a closed session, the completeness of the cleanout is less dependent on your actions.
At the end of either session, it is a good idea to sit on the toilet and do occasional hula moves to be sure your are not retaining water. If you have a long trip home, consider a disposable diaper too. Accidents do happen.