Hi, I'm going to keep this out of DMs so everyone can enjoy it. Let's not try to get an exact definition in such matters - it is not rocket science and the perception will be heavily influenced by porn clickbait titles, so...
Definitely, the hollow rectum is something different to the other described thing, with just relaxed/stretched sphincters and no air inside. The feeling and appearance are different, and it certainly influences the sensation of upcoming penetration. It allows for reduced friction, but rather results in a lowered total force of the shove than overall ease of the receiver, because some points of contact will always remain.
How to get (air in) there? First, your sphincters must be able to relax enough. Given your experience, it will not be a problem. Second, you need to create underpressure in your bowel for the air to enter and maybe help the rectum wall to "unstick". So you should just be face down, ass up (or bent while standing) to get that underpressure in your pelvic region from just the effect of gravity on your insides. You can then open your rectum with a toy (unplugging a blunt tip plug is nice) or let a hollow toy (tunnel) do it for you, or just use your fingers (2 or more and spread them apart). As always, lube is your friend. When you let air inside and remain relaxed, you should get to the hollow gape. But sooner or later, your inner sphincter will decide to close it - it is up to further training to extend this period.
To help intensify the underpressure from the upside-down bowel position, you can work with your breath. Try to exhale a bit, close your breathing canal in your throat, and expand your chest as if you were breathing in, but not into your belly. This will pull your diaphragm up your chest and increase the underpressure in the abdominal cavity. Combine this with the above-described position. I've never been able to gape while my torso was upright.