I have recently tried various strengths of glycerin enemas. Granted, they are pretty low in concentration but I have wanted to gradually step up in concentration.
My first couple of enemas with glycerin added also contained soap (I usually use Ivory) and it was difficult to tell if the small amount of "cramping" that I experienced was due to the glycerin or the combined effect of the soap and glycerin. Increasing the concentration up from three to five percent produced a bit more of an effect.
There is another story around my most recent purchase of glycerin (I knew I wasn't the only one buying glycerin as a "skin protectant") along with other supplies of soap, and sodium bicarbonate. If you hang around where they sell glycerin, you might meet the most interesting people.
My most recent experiment was at 10% strength (200 ml added to 1.8 L of water, no soap). I usually take a larger volume enema but I have found an interesting effect using lower volumes with my new double balloon nozzle. I'll write about that that elsewhere. After mixing the solution (warm water) and carefully filling the bag, hose, Higginson pump, and nozzle to purge as much air from the system as I could, I hung the bag from one of the four posts on my bed. A thick towel on the bed, a clear path to the toilet, toilet seat up, and lubricant at the ready, I laid down on the bed, lubed the nozzle and inserted it.
Now, I've read what is possible with increasing strengths of glycerin so I wanted to make sure the nozzle was inserted, properly inflated and well-seated. I also had a plan for how to roll out of bed in case I had more "urgency" than I thought I could deal with.
Since the elevation of the bag was much lower than I typically use, I didn't expect a high flow rate. I set the valve to a fairly slow rate. It took 10 minutes to completely empty the bag (I had to use the pump to drain the entire two liters) and close the valve.
I observed the first rumblings at the five minute mark (about one liter in) that seemed different from the "normal" enema experience. These continued to intensify as the liquid filled deeper and deeper. When I closed the valve at 10-minutes, I was comfortably filled with a nice warm liquid and the periodic contraction and pressure build against the balloon nozzle. The nozzle was holding with no leakage. For the next five minutes, the contractions were stronger than I normally experience but I was able to allow the pressure to build and then feel the sudden internal release of the muscles and the "backflow" of the solution back up into my colon. This muscle action also has the benefit of being very stimulative of my prostate.
At the twenty minute mark, the pressure of each wave was getting stronger and more frequent. It wasn't constant pressure and I wouldn't say that they were cramps, but it definitely was stronger than I normally experience. As I approached the 25-minute mark, the pressure to expel was getting very intense and even though there was no leakage, I was getting concerned about the holding power of the balloon. The intensity of the spasms also provided a great deal of internal stimulation that was quite pleasant but even that was beginning to get overwhelmed by the urge (need) to expel.
I carefully rolled out of the bed, unhooked the empty enema bag from bedpost and carefully shuffled into the bathroom. Gravity increased the pressure but not in a panicky way. Still no leakage. I hooked the bag on the shower curtain rod and carefully positioned the nozzle inflator bulbs between my legs as I settled down onto the toilet. The pressure was incredibly strong as I released the valve on the outer balloon. It still held. There weren't really distinct waves of pressure at this point...just a constant pressure that seemed to emanate from everywhere. I barely touched the release valve on the inner balloon when it fired out nearly fully inflated along with a torrent of the enema solution. Long, hard contractions discharged the contents of my colon.
These contractions lasted for about 20 minutes and when I finally applied a rinse (about 2.5 liters) and expelled the rinse water, they subsided. I'll do some more at 10% concentration. When I have more time, I plan to increase the concentration in steps (15, 20, maybe 25 percent depending upon my colon's reaction). I can't see doing this every time or at very high concentrations.
However, at 10% concentration I found the experience quite pleasurable.