To be in the zone and focused on a task or during an event. People put in stressful situation who are rolling with the punches and not getting hung up on things are switched on. First responders, military personel and those who deal with dangerous and stressful situations are known to become "switched on" when faced with an event.
A joke often used in the YouTube comments section, in which someone says "I switched to porn when my mom walked in, it's easier to explain."
People who use this joke think they are calling the video weird in a funny or original way, when in reality it's been plastered all over YouTube thousands of times. Someone who says this is often referred to as a faggot.
Person A: "Lol, this video is so weird I switched to porn when my mom walked in, it's easier to explain!"
Person B: "This joke has been viewed thousands of times and as such has been rendered completely humorless. I imagine even the dumbass that came up with the joke is weeping right now."