I've never heard this mentioned, and I can only speculate as to why this works, but for me, doing sit-ups, particularly decline sit-ups with my legs far raised, helps temporarily reduce spasms and pain for me.
It might work by relieving the pressure down there, encouraging blood flow to the region, forcing my internal organs to contract upwards from the motion, or psychosomatic magic, but it's a go-to for me since I exercise every day.
Depending on your fissure, this may no be possible. But it works consistently for me. Does it have the same affect on anyone else? It's not as nice as a sitz bath, which for me is a sitz shower since I have an adjustable shower head. But it consistently lessons or temporarily stops the pain of spasms for me.

So, your explanation sounds plausible, great to hear even a newer way to stop the pain.
GL