My first experience with a deeply inserted colon tube was in my 20s during a colonic and enema session with Mildred Burgess in Boston. She used the "jerking off" method described by Switchablesusie. She also did some twisting and turning and, as Susie said, lubrication is important, be careful because you don't want to irritate your anus, or worse still, cause a hemorrhoid.
It took me quite a while to find just the right colon tube when I tried to do them at home. A friend had purchased all the enema equipment from a nursing home that included several 38 French tubes manufactured by Davol. They had a combination of diameter and flex and you could literally feel it almost spring itself around the sigmoid flexure, and you were home free from there.
It wasn't just me, my wife was into deeply inserted tubes at the time, and the tube and the senation were the same for her, that tube would snake perfectly around the flexure. We used it for about a decade until the rubber started to deteriorate. The slickness of the tube was just right too.
Being the moderate size it was we were often able to get it in deeply without a pre-enema. Let's just say there were a lot of fantastic orgasms thanks to this tube.
After the tube deteriorated every other tube we tried fell short (pun fully intended), but, thanks to eBay we purchased both sigmoidoscopes - rigid and flexible colonoscopes. Great fun, and you literally "drive" it through with the tip steering controls, of course you get a nice look around too. It wasn't until SiliconeNozzles came online that we were able to find a tube as much fun as the old Davol.
We tried several until we found the right combination. Only problem with the silicone tubes is a slightly tacky surface needing a lot of lube. As Susie says, particularly with the large diameters, without a clean out first you may be pushing against a lot of feces. But the sensation from some of the larger diameter tubes is incredible.
My first tubations by Mildred were following a colonic, but as it turned from colonic sessions to enema play there were times when I was tubed when I was full, and getting the tube around the poop was something Mildred could do like no one else.
Another trick, tape the tube to your leg and expel around it, doesn't always work, but it can be fun. I have had a continuous flow going sitting on the toilet the bag on an IV stand and Mildred or a playmate keeping the bag topped off. Lying in the tub, which I would do at home, is even better ! I was always amazed that Mildred kept a professional demeanor while providing some of the the most amazing enema play.
Only thing better was when I met Valarie at Grand Dames who could do nearly everything Mildred would do, but it was all about sex, and always a happy ending.