It's an involuntary gesture and I'd bet it has happened to all of us: You want to point out something on your slides but instead of pointing to the projected slide that the audience sees, you're pointing to your laptop's screen or the preview monitor.
Granted, there are worse mistakes you could make in your presentation. Like looking at the projected slides and turning your back to the audience all the time. But still, that little misdirected gesture can be irritating.
How do you avoid it? One thing that obviously does not work is moving the speaker's screen. I've had the pleasure of attending TEDGlobal>Geneva recently and it took me a while to realise that they didn't have a preview monitor on the stage. It didn't take me long to figure out where it was, though, just by watching the speakers involuntarily pointing at it: It was in the back of the room, hanging from a balcony. So this well-meant attempt didn't work. In fact, I think it even made it slightly worse. When the audience can see the preview monitor while the speaker is pointing at it, it's clear what's happening. If they can't see the screen and the speaker makes a gesture pointing towards something in the back of the room, it could be misinterpreted as asking the audience to turn around. Thankfully, that didn't happen at the event I attended.
In this panorama shot , the preview monitor is the small bright white rectangle on the left.
So, what can you do? As with all involuntary things, first of all you have to be aware that you're doing it. It's the same with the "um" and "ehm" sounds speakers often make. Watch a recording of yourself giving a talk and watch out for that gesture (or sound). See if you can catch yourself doing it when you next rehearse your talk. Ask someone to watch out for it for you. Agree on a hand sign to have it pointed out to you when you rehearse in front of a friendly audience.
There's no magic bullet to prevent that gesture. You will have to practise, practise, practise.
Please email me for details.