Unrounded Multiply is one of those names that are really confusing unless you know the history behind it.
I think of it as "Cut", which for most people would be a more obvious name, and I'm going to add a new function to reinforce that. Let's imagine there is a new function named Cut that does this...
When you press Cut the first time it marks the start of the cut. When you press Cut a second time, it ends the Cut and the section between the two points is sliced out and becomes a new layer.
What makes Cut a multiply is what happens if you let the cut continue beyond the end of the loop. The Cut has to continue somewhere, it can't loop back to the beginning and overlap itself, so it's like it automatically glues another copy of the loop onto the end, and continues the cut. This cut extension will continue as long as you let the cut continue past the loop end.
That's why it is basically a special form of Multiply. It behaves like a Multiply, with some differences about what happens when you end it. A Multiply will always be "rounded off" so that the layer will be an even multiple of the original layer's cycle size. That allows it to continue playing in sync. An unrounded multiply means to stop the multiply immediately when you press the button, the resulting layer may be of an odd size and not necessarily in sync. But it is basically that cut between two points.
To get an unrounded multiply, you start with the Multiply function and end with the Record function. That's another point of confusion. I often say Unrounded Multiply but it's assumed you know how to get there with two different functions Multiply then Record. The hypothetical Cut function, would do both edges with one function. The main reason Multiply/Record exists is because not everyone has a massive footswitch with enough room for a Cut button. So you can overload the Multiply and Record buttons to accomplish the same thing.
When you're in a script though you've got an infinite number of "buttons" so you can call it what it is, and it looks clearer.