That feeling you get when you see something exciting.

Ex. Did you see the new Batman v. Superman trailer? Commence heavy breathing.

When you hear someone masturbating while having a conversation via either phone or any other forms of inter-communications e.g. Skype.

When someone sees something that relates to something they like, so they start getting excited.

Shes playing with herself

When someone is breathing as hard and heavy as a dad who has just found out that his daughter has a boyfriend and he's coming over.