When a guy is turned on, but not quite ready to grab a couch pillow to awkwardly conceal his arousal- an erection that is safe for public consumption- i.e. not pitching a tent, but beginning to feel aroused.
When that girl with thatfine ass bent over right in front of me to adjust the spoke nipples on her Fuji bicycle, I chubbed up a bit.
When a member of your own fireteam gracefully and wonderfully strafes in front of you at the moment you fire your Gjallarhorn rocket leading to your sudden demise followed by them moving to the side like nothing happened!