Lately I have had success taking 4L standing up. Seems crazy but it works for me. I hang the bag about 3 feet above my waist and let it rip full speed. I usually get all 4L in on my second fill. First fill is usually 2L with Dr Bronner’s peppermint soap to get a good clean out of the lower bowel. Also taken standing up.
Here is my thought on taking an enema standing up. The way I see it is the descending colon will hold most of the water as it's going uphill and therefore distend it more. My gut tells me (LOL) that anatomically that's probably not great for your colon. Knee chest, all fours, on your back, keeps your colon flat and that allows water to flow more evenly inside you.
In public, usually next to the freeway, naked, no bowel movements in previous couple days, face down ass up, 1L water with glycerine, then a big syringe full of milk and molasses. Hold as long as possible and push hard when it comes.....for best show. Then diaper and head home for soapy enemas in simms and then a couple clean out large volume baking soda and salt on back with lots of deep fisting in-between and after....and then, finally my chastity cage comes off, finally, a 1gal porbiotic enema on back in a hot bath while she gives me a slow blow job/hanjob which ceases entirely if I expel any of the enema.
The first two or three enemas are performed on the left side in the Sims position with the right leg pulled up to the chest, lying on the shower floor to avoid any leakage. Subsequent enemas can be performed lying on the back or changing positions, rolling from the left side to the back and then to the right side. The most interesting thing is that in the knees-chest position, I can perform fewer enemas than on the left side. As a child and teenager, I used the upside-down position, standing on my shoulder blades and leaning my back against the wall, but I've already forgotten what it felt like to perform these "acrobatic enemas." I'll try to remember them during future sessions.
Knee-chest works very well for me -- actually that is the only position I can take any 'real' enema (say more than 1 litre).