How can I activate Mac speech without pressing a key?
This is quoted directly from the Speech Recognition Settings preferences:



Listen only while key is pressed:


  With Speakable Items turned on, you can also select this method by
  speaking the command “Turn on push-to-talk.”


Listen continuously with keyword:


  Use the “Keyword is” pop-up menu to change when you need to speak the
  keyword. Use the Keyword field to specify what the keyword is.