[ruby-core:23119] internal_encoding() and write()

I'm I correct that internal_encoding isn't needed at all when writing  
to an IO?  The conversion seems to happen automatically.  For example,  
this:

   # encoding: UTF-8
   open("data.txt", "w:UTF-16LE:UTF-8") do |file|
     data = "My data
     file.puts data
   end

seems to produce the same output as when I leave out the second  
encoding:

   # encoding: UTF-8
   open("data.txt", "w:UTF-16LE") do |file|
     data = "My data
     file.puts data
   end

Is internal_encoding() for reading only?

James Edward Gray II