I think that's actually a Gem feature. Gem installs wrappers for the gem 'binaries' which include something like this (taken from the passenger-install-apahce2-module example):

```ruby
require 'rubygems'

version = ">= 0"

if ARGV.first
  str = ARGV.first
  str = str.dup.force_encoding("BINARY") if str.respond_to? :force_encoding
  if str =~ /\A_(.*)_\z/
    version = $1
    ARGV.shift
  end
end

gem 'passenger', version
load Gem.bin_path('passenger', 'passenger-install-apache2-module', version) 
```

Not sure how long it has been in RubyGems, but a good number of years at least. Here's [another reference to this](http://gistflow.com/posts/497-running-a-specific-version-of-a-ruby-executable)