how many words to use?
Unco is good for:
Roll from the core
Balancing and staying aligned while being thrown off course by the recovering arm, or the pulling arm
Connection your catch and pull with your body twist/roll
Breathing while not disturbing the process.
Because you are more concentrated one one side of the body you can focus on whats happening better.
Unco or one arm swimming is a very difficult action to do perfect. Even the olympic swimmer cant keep perfectly aligned and struggles a bit if you watch her in slowmo.
The beauty is that its very close to the starting point of swimming, that is being in balance rotationally and between front and rear.
Now you add disturbing actions and you have to stay balanced and also make sure you are making forward progress while staying aligned.
With every action you can focus on the effect of that action.
How does lifting of the arm out the water effects my rotational balance and where does the body sink during that action? How does the body react on a fast recovery, on a slow recovery? How does the lifting of the non working arm help rotation? Can I use more core and legs or a smoother recovery to do that work in the water?
What part of the body is bending a bit instead of only rotating around the axis? In what sequence connect all the bodyparts if I imagine them to be seperate parts, only allowed to rotate around a spit? Can I add abit of that bending and twisting to my advantage without creating too much drag, or loosing the rotation around a spit idea too much?
How do I make the whole cycle smooth? How can my legs help rotation a bit?
At what time do I add a bit of shoulder twist and reach, and how do i set up the arm in the water so that the following actions dont cause the body to be steered off line or the body starts to bounce?
Because there is only one arm thats anchoring, its more important to get the connection with the water and all the following actions in the body timed just right. You are having a one cilinder engine that you are trying to operate as vibration free as a 2 cilinder engine.
If you can do that, your 2 cilinder engine will run as smooth as a 4 cilinder engine.