Opening up the second sphincter as it’s often called, is for most just as challenging as it was learning to take a fist. Playing deep can evoke feelings of discomfort or pleasure depending your level and care to avoid injury.
The Pubo-Rectal sling (second sphincter) is a loop of muscle that in its natural state, holds the lower intestine up to create the boundary between the rectum and the sigmoid. In essence, when you have a bowel movement it will relax and allow everything to straighten out for elimination. So the challenge with depth play is to learn to allow it to relax and straighten at will so things can pass through more easily.
There seems to be some consensus in the things I’ve read that there is a sphincter of sorts in the Sigmoid as well. This would indeed explain the intense feelings of pleasure or discomfort that I encounter when playing at about 14-16 inches deep.