I'd say quite the opposite. With many people it seems it is impossible to hold back, and they just have to run; the severe and sudden urge is often caused not by any real need but rather due to inflammation or damage to the nerves that control the two sphincters at the anus. From what I could gather when once upon a time I was reading up on how the anus operates (
need to get out more
) the nerves are under semi-autonomic control. This means, as we all know, we have some control of what we do with them. But how much control we have is modified by practice - so if we practice lots at keeping the muscles clenched and fighting what the nerves are saying, then the strength of impulse that they send and the answering response from other parts of the body may well be mitigated. There is a feedback mechanism by which you can "teach" your anal sphincter and gut muscles to behave.
In other words, if you keep on trying to hold on for a few seconds more, then in time your gut might learn not to be so hasty at
opening the floodgates.
It's a fine theory...it works that way with me, anyhow. What I find is that the more I delay going, the stronger the impulses get, until eventually I have no choice - but over the years my control has improved.
I think I might just be lucky. Like I say, it seems this doesn't apply to everyone. It will no doubt depend on how severe your inflammation is.
One thing you can be sure of, unless you already have a really severe weakness like a fistula, you are not going to cause a rupture. The spasms you feel run along your gut, that is how the muscle cells are aligned, in order to push materiel along the gut - the violence of diarrhoea coming out is not like a chemical reaction boiling under ever more pressure, it is more like a syringe effect. Turbocharged peristalsis. Very much in one direction only.
Good luck with the experiment...!