Barge-in types in VoiceXML

 Hi,
      In the VoiceXML 2.0 specifications, the barge-in types enumerated are:

      energy, speech and recognition.

	The description for both energy and speech reads the same:
	"The prompt will be stopped if speech or a DTMF tone is detected."

	I am not sure how would the distinction work and also, conceptually,
what is 
	the difference between energy and speech in this context? 

 Thanks,
 Ranjan