I see face- and mouth-covering behavior in public speaking situations more often when people are sitting than standing, during interviews or discussions where the setting may feel more casual than a presentation. But it happens in all presentation settings, and it's a problem with a quick fix.
It comes in the form of a man stroking his mustache or goatee, someone scratching their nose or resting their chin in their hand, and a million other little distractions. It's an unconscious habit, possibly a self-soothing or protective one, and most people don't realize they're doing it.
When you put your hand anywhere on your face while speaking, one of four things is likely to happen:
1. Your movement becomes distracting and takes away from your content.
2. Your audience can't hear what you're saying because your hand muddles your voice.
3. Your audience can't see what you're saying because your hand covers your mouth.
4. Your audience believes you're embarrassed or hiding something because you hide your mouth.
When you are speaking, the audience needs to see AND hear you. Nonverbal communication (gestures, facial expressions, tone of voice, emotion, etc.) adds depth to words and helps your audience better comprehend your attitude, personality and message.
It's also easier to understand a speaker's words when you can see what her or his mouth is doing; some words that sound alike can get mixed up if we can't see your mouth, especially for people who are more visual in general or for people with hearing impairments. (Speaking for myself, one reason I don't like talking on the phone is that I can't see the other person's face. I don't feel that I can fully grasp a conversation without seeing someone's mouth.)
This is an example of why it's harder to keep your audience's attention when you're giving a teleseminar or webinar, where the audience can't see your face. Not only is there a lack of visual stimulation, but most of your ability to communicate nonverbally is absent.
If you have a tendency to put your hand on your face or near your mouth, ask someone to watch you next time you speak, either onstage or in a meeting. Or videotape yourself to get a sense of your movement. If you're covering or obscuring your face or mouth in any way, become aware of this habit and start weaning yourself off of it.
Try breathing and relaxation techniques before you speak if you feel this habit is anxiety-related, but also know that awareness is your first line of defense. When you feel your hand drifting up toward your face, ask yourself if it's a necessary movement or a habitual one, then act accordingly. What do you have to hide?