What happens while holding an enema has long fascinated me--especially, how the state of relaxation, or lack thereof, affects comfort and flow rate.
Several of you have mentioned relaxation and submission. Relaxing and submitting to overcome the urge to expel, and then allow the entry of still more enema solution is a highlight of my enema experience. It's especially exciting to feel the flow resume as I relax, yet challenging because I know that I may be approaching my limit. Balloon nozzles are pretty much bullet proof, and therefore take away this particular pleasure.
Then there is the role of sexual stimulation, which several have also mentioned. Long ago I used to give myself brutal enemas. They were quite hot and used laundry detergent as the soap. They were so rough that I would need to sleep for hours afterward to recover. But the real point is not the ignorant use of such strong enemas, it's how sexual stimulation affected the course of events.
I would take these enemas (2 qt) while laying on my stomach with a rolled up towel under my privates, and masturbate as the enema entered. It was a race between the sensation of the horrid enema and my orgasm. The orgasm never failed, and then the strangest thing happened. I could EASILY take the remainder of the bag after orgasm. It was as if the foul solution had been replaced by the lavender water. So I would complete the enema, expel, and then nap for several hours to recover. For some time the tension between the painful enema, the sexual summit, and then the sweet coast to completion was very attractive.
I would love to understand the mental and physical processes that could so totally alter the sensation of those enemas.