Somebody does this when they are in an awkward situation, feel guilt or are under pressure. They also make a strange noise and tug on their collar to indicate that they are uncomfortable. This often happens in "The Simpsons"
Homer did the collartug when he was giving a speech and everyone booed
When finding your one-night stand/booty call/random hook-up has a tiny penis and out of sympathy you give him a hand job. This is often the last encounter before deleting his number from your phone.
After Frankie found her bar fuck for the night had an incredibly small penis, she gave him a pity tug and never spoke to him again.