Were you raised to be a "good" kid? If you were, and the instructor was boring, it highly likely that you wouldn't want to give the 'wrong impression' by not listening, but would it really be the wrong impression? There are of course other reasons you would want to appear to be listening, but I've found that if a teacher/instructor is boring you, and you decide to go into the material while they're talking, they'll either think you're interested in the material, but not his voice. Wouldn't that be exactly what you should send to receive anyway?
For me, it usually doesn't matter when, how, where, or why I do PhotoFocus in whatever fashion I see fit. However, I find that it's usually best, at least for me, to be immersed in the material through a constant full out state during activation, taking in everything I can for at least 20 minutes straight with nothing else around me.
-youngprer