The way it would work is, select a unit, then run the command and it reduces their sentence time to 0, so that on the next tick they get released.

The motivating example I had for this is that I was lagging behind production orders. My militia commander was sentenced to a long-ish prison sentence. I then sent out the squad on a mission but realised the squad leader was in prison, while the rest of the squad was off map. They can't return until the mission is complete, but the mission cannot start until they all leave the map. Hence, because of this guy being in prison, they are stuck out there waiting for his prison sentence to complete.

There might be other instances as well where it might be useful to the player to commute the sentence of a convict as well.