When the button is pressed, normally, although the Select command being on a line before may complicate things, as well as the button changing which sides are source/dest on the first line.
I would use scripting instead for this, as you can be more explicit about what you're doing, and also completely avoid the source/dest switching which will make the file display borders flash. (The same advice will apply to a lot of situations where you want to do something against the grain that acts on the destination instead of the source. Scripting lets you do that directly, while using non-script commands requires kludges.)
As an aside, I'd also recommend having a browse through Formatting tips specific to the Opus forum for how to format commands/code on the forum so your * characters don't turn into italics/bold etc. (I've edited both posts in this thread to fix them.)