This is where the sigmoid kink begins. Overcoming this hurdle requires some patience, practice and mindfulness.
We do it like this:
Apply plenty of lubricant to the anus, penetrating deeply with your fingers and making it lubricated; apply lubricant to the first 15 to 20 cm of the intestinal tube, insert the intestinal tube slowly turning, after about 15 to 18 cm you will feel the first resistance.
Pull back the rectal tube a little, start the enema slowly, slowly inserting the tube with back and forth movements. This usually succeeds at the very moment when the liquid runs from the rectum into the large intestine.
The tube can then be lowered further into the depths of the colon.
Here are two links where about 30" colon tubes are inserted:
https://biguz.net/watch.php?id=2033906&name=jake%27s-colon-tube-enema
https://pornzog.com/video/8499291/annie-s-enema/
It works similarly with colon snakes, only that no enema liquid is used here. We use a small syringe to inject about 30 to 50 ml of lubricant into the rectum beforehand. And then the game begins. So far it has always worked...