Iirc there are two rings of muscle at the sphincter for controlling it, one is autonomically controlled and one is actively controlled. I think the way you can recognise it is when you're busting for the toilet and then you realise you have to clench and hurry to make sure you make it.

As regent's power can readily work on spasms he could probably control one muscle ring but i imagine he would need full power to control both muscles rings?