There is a 'second ring'- kind of.
Between the rectum and the sigmoid colon. There is a change in the smooth muscles that form the wall of the colon and that of the rectum. There is a constriction here that is not built like a bona fide sphincter... but can be tighter or looser depending on how relaxed you are, or aren't.
There is also a pretty sharp KINK in the plumbing at this point.
The combination of the kink, and the muscles lining the sigmoid can effectively pinch off flow, or penetration, between the two pretty effectively.
Anyone who has given themselves an enema has experienced that you can insert a certain volume of water.... till minor discomfort... but then you can insert considerably more.-- this is because water will initially fill the rectum- ballooning it out till its full... and only then develop enough pressure to push past the kink.
When expelling the water, you get the reverse- the water in the rectum comes out all at once in a rush... but if you got enough in to get water up into the sigmoid or beyond... it will take a little more time to work out the additional fluid.
( you also see the same thing in taking a dump- an initial easy release of the material in the rectum- and then a pause until the material past the kink can be let down into the rectum for elimination --- or how a full bladder can create a situation in which you can shit... but then have to empty your bladder in order to get more shit out. The full bladder and squeeze the 'kink' tighter and prevent the rest of the stool from passing until that pressure is relieved.)
In deep ass fucking- your rectum can stretch out about 6-7 inches before you 'bottom out' by hitting that kink in the tube.
Hitting that kink with something longer than you are deep will cause an involuntary muscular contraction, pinching the 'ring' down tighter and resulting in significant pain.
However- your guts are pretty mobile- and if you practice with a long dildo... you can figure out how to angle your hips and the dildo such that you can 'straighten out' that kink enough allow for far deeper penetration.
Once you get the dildo pressing gently agasint the constriction, all you have to do is maintain a gentle but steady pressure, and the muscle will relax, and allow entry.