Knee-chest is ideal for a full cleanse, or other large volume enema, because it is an advantage in retention.
It makes gravity your friend, not your enemy.
It IS a little awkward when you self-administer.
If you have any bad knees or anything like that, you need to put a pillow under your knees, or use Simms position.
Also if you are larger in the bust line, you'll probably need a pillow under your breasts, so putting another one under your knees helps balance things out.
Relaxation is so important for a good effective enema, and Simms Position, that is left side, with the right hip =drawn up to where your knee almost touches your breast, left hip straight, left knee at a 90 degree angle. is the easiest to relax in. It also helps "open up" your rectum enough that the nozzle will slide in easily, without opening it too much so that you leak.
Those are the two real "enema" positions. It needs to be one or the other.
Also, I only discovered this when my husband and I started playing around with foreplay enemas.
Some men, at least, find it arousing to see a woman in the Simms position. Not sure WHY, but whatever works.