This unfortunate piece of advice is offered in the hope of allaying nervousness while also making the audience think you’re looking at them.
I’ve also heard it suggested to find one friendly face and talk to that person.
Both of these have serious downsides and avoid the actual solution.
Looking at people’s foreheads may fool the people you’re looking at, but you’ll miss the calming effect of looking in their eyes. You’ll also miss the real connection that tells you whether or not they’re with you. In addition, you’ll lose the immediacy of conversation; your voice will become less energetic and your gestures and body language will fade.
Choosing one person in the audience to look at, instead of including everyone, will also rob you of visual feedback and dilute your connection with the audience. Along with making everyone else feel unneccessary, you’ll make that person you’re looking at uncomfortable.
So, for real connection and real comfort, look at (and see) each person long enough to connect. And take your time having a conversation with the entire audience — one person at a time.