The short answer is find another way, any other way, to keep students from falling into the dead zone. I like to have a two minute class stretch then get back to work. Almost any training class can have some sort of group assignment or even a short role playing game that helps solidify core concepts, and keeps people moving. The game format is pretty basic, and the questions are really a lesson review in disguise. Contrary to popular assumptions, students generally a short video.
Students look forward to the change of pace a guest speaker brings.
Often a new person can lecture straight through the dead zone without losing a single eyeball. And don t forget field trips. Sometimes I just take them a few feet down the hall to see our wiring closet. A good old fashioned quiz also works wonders. Along with making the quiz open book, you can even let students collaborate.