Why:
A) When I'm about to perform an act of utter clumsiness, I see the glass begin to topple for instance, do I perform an exaggerated intake of breath?
B) When I witness an absolutely unbelievable magic trick, do I hold my hand to my mouth?
I know it's not just me, we all do it, but what could possibly be the evolutionary benefit of either / both?
Completely unscientific explanation.... in the cave day we needed to be able to instantly start running or climb a tree to avoid predators. When our brain receives a perception that it doesn't understand instantly it puts the body in 'fight or flight' mode. Epinephrine is released from the adrenaline gland and gives the body a 'rush'. To enable the body to do react to the stressful situation it needs plenty of oxygen in the blood hence the gasp. The body then freezes untill the brain has processed the situation and decides what action to take. This all happens in a second.
The hand over the mouth is just politeness, just as we (some) cover the mouth with the hand when yawning. You need to give me at least one star for imagination!
Some say it's just "manners", not to leave your mouth gaping - and claim only women do it ........
Or, you could go for the other version : emotional / verbalisation suppression, so as not to draw unwanted attention to yourself or the event, or attract the scary thing to you.
Other primates have been seen to use their hands to conceal part of their facial expressions.