diff --git a/middleman-core/lib/middleman-core/extensions.rb b/middleman-core/lib/middleman-core/extensions.rb
index 47157122..70d3026c 100644
--- a/middleman-core/lib/middleman-core/extensions.rb
+++ b/middleman-core/lib/middleman-core/extensions.rb
@@ -3,72 +3,80 @@ require 'middleman-core/extension'
 module Middleman
   # The Extensions module is used to handle global registration and loading of Middleman Extensions.
   #
-  # The application-facing extension API (activate, etc) is in Middleman::CoreExtensions::Extensions in
-  # middleman-core/core_extensions/extensions.rb.
+  # The application-facing extension API ({Middleman::CoreExtensions::Extensions#activate activate}, etc) is in {Middleman::CoreExtensions::Extensions} in
+  # `middleman-core/core_extensions/extensions.rb`.
   module Extensions
+    @registered = {}
+
     class << self
-      def registered
-        @_registered ||= {}
-      end
+      # A hash of all registered extensions. Registered extensions are not necessarily active - this
+      # is the set of all extensions that are known to Middleman.
+      # @return [Hash{Symbol => Class<Middleman::Extension>, Proc}] A directory of known extensions indexed by the name they were registered under. The value may be a Proc, which can be lazily called to return an extension class.
+      attr_reader :registered
 
       # Register a new extension. Choose a name which will be
-      # used to activate the extension in config.rb, like this:
+      # used to activate the extension in `config.rb`, like this:
       #
       #     activate :my_extension
       #
-      # Provide your extension module either as the namespace
-      # parameter:
+      # Provide your extension class either as the second parameter:
       #
       #     Middleman::Extensions.register(:my_extension, MyExtension)
       #
-      # Or return it from a block:
+      # Or better, return it from a block, which allows you to lazily require the implementation:
       #
-      #     Middleman::Extensions.register(:my_extension) do
+      #     Middleman::Extensions.register :my_extension do
       #       require 'my_extension'
       #       MyExtension
       #     end
       #
       # @param [Symbol] name The name of the extension
-      # @param [Module] namespace The extension module
+      # @param [Class<Middleman::Extension>] extension_class The extension class (Must inherit from {Middleman::Extension})
       # @yield Instead of passing a module in namespace, you can provide
-      #        a block which returns your extension module. This gives
+      #        a block which returns your extension class. This gives
       #        you the ability to require other files only when the
-      #        extension is activated.
-      def register(name, namespace=nil, &block)
+      #        extension is first activated.
+      # @return [void]
+      def register(name, extension_class=nil, &block)
+        raise "Extension name must be a symbol" unless name.is_a?(Symbol)
         # If we've already got an extension registered under this name, bail out
-        raise "There is already an extension registered with the name '#{name}'" if registered.key?(name.to_sym)
+        raise "There is already an extension registered with the name '#{name}'" if registered.key?(name)
 
-        registered[name.to_sym] = if block_given?
+        registered[name] = if block_given?
           block
-        elsif namespace && namespace.ancestors.include?(::Middleman::Extension)
-          namespace
+        elsif extension_class && extension_class.ancestors.include?(::Middleman::Extension)
+          extension_class
         else
           raise 'You must provide a Middleman::Extension or a block that returns a Middleman::Extension'
         end
       end
 
-      # Load an extension by name, evaluating block definition if necessary.
+      # @api private
+      # Load an extension by name, lazily evaluating the block provided to {#register} if necessary.
+      # @param [Symbol] name The name of the extension
+      # @return [Class<Middleman::Extension>] A {Middleman::Extension} class implementing the extension
+      #
       def load(name)
-        name = name.to_sym
+        raise "Extension name must be a symbol" unless name.is_a?(Symbol)
 
         unless registered.key?(name)
           raise "Unknown Extension: #{name}. Check the name and make sure you have referenced the extension's gem in your Gemfile."
         end
 
-        extension = registered[name]
-        if extension.is_a?(Proc)
-          extension = extension.call
-          registered[name] = extension
+        extension_class = registered[name]
+        if extension_class.is_a?(Proc)
+          extension_class = extension_class.call
+          registered[name] = extension_class
         end
 
-        unless extension.ancestors.include?(::Middleman::Extension)
+        unless extension_class.ancestors.include?(::Middleman::Extension)
           raise "Tried to activate old-style extension: #{name}. They are no longer supported."
         end
 
         # Set the extension's name to whatever it was registered as.
-        extension.ext_name = name
+        extension_class.ext_name = name
 
-        extension
+        extension_class
       end
     end
   end