This plugin is meant to simplify a task I've found too common in my workflow: switching between a single-line statement and a multi-line one.
I usually work with ruby and a lot of expressions can be written very concisely on a single line. A good example is the "if" statement:
puts "foo" if bar?
This is a great feature of the language, but when you need to add more statements to the body of the "if", you need to rewrite it:
if bar? puts "foo" puts "baz" end
The idea of this plugin is to introduce a single key binding for transforming a line like this:
<div id="foo"> bar </div>
And another binding for the opposite transformation. This currently works for various constructs in ruby and erb, and for tags in html/xml.
For more information, try
:help splitjoin, or take a look at the help file
For more examples and corner cases, you can explore the "examples" directory here: examples. It's not present in the downloadable zip file to avoid cluttering your vimfiles with useless stuff.