I have the same exact experience as the OP.
In my case, the last thing I want to do is just relax and let it go. Having an accident is absolutely not what I want, and if I can just get through the spasm, I'll often have 10-15 minutes to get to the toilet before a spasm hits again.
I don't want to take the time to change my brief. I don't want to pay the cost of the extra brief - the ones I wear are about two dollars each. I don't want the worry that I'll smell like pee, which I always stress about after an accident no matter how hard I work to clean myself up. I don't want to have to deal with the embarrassment of disposing of a wet brief, especially when I'm at work.
The briefs I wear are plenty capable of absorbing my biggest accident without a problem - I actually wear much more absorbent ones than I truly need because they're the only ones I've found that don't clump after a few hours and leave me feeling like I'm sitting on golf balls. But there are so many reasons that I don't want to just pee in them. If nothing else, I find it gross to sit in my own body waste, even for a few minutes before I can change.