I think that there may be an explanation for what jackdavinci has felt, but maybe has not understood.
In general, pressure on the anus (for gas or solids) is generated by peristaltic motions of the colorectal walls. This keeps happening during, it’s what your gut does. Sometimes, as material moves closer to the anus, these peristaltic motions create the feeling that drives us to evacuate or eructate. But in general, the bowel is not full by any means. So a tight clenching of the anal sphincter can be enough to hold it closed against the pressure of the peristaltic motion, and can (because it is semi-autonomous) shut those motions down temporarily. It does feel like you have “pushed back”. You haven’t, and the material that needs to be expelled is still there. But gases can leak out the more relaxed anus slowly under weaker peristalsis, or it will push out the fecal material with a gush when you let it all out.
But nothing goes back up past the sphincter to the large intestine, gas or solids, unless you are very sick. And nothing you can do will change the odour for the better. Of course, holding the gas in longer may act to increase … potency.