The document discusses the Decent Exposure gem, which provides a DSL for defining controller instance variables from simple blocks. It allows defining a default exposure that will be used if no block is provided. When expose is called on a controller, it defines a method, makes it a helper, and hides the action. It uses instance_eval/instance_exec to invoke blocks within the controller context. The gem source code is also discussed, showing how it implements default exposure and the expose method using up to 2 block invocations.