When during a conversation, someone might possibly refer to something regarding a Noodle Incident or something less severe, but still might be too much information. Then before the recipient of the conversation even asks, the person with the usually embarrassing story preemptively pops the reason, usually after saying "Don't Ask".

i.e. Said user changed his IM handle to "Toenail Fungus":

Toenail Fungus: Don't ask about the nickname I changed to. It's because I have this yellow toenail. I grinded it with a pedicure kit down to the skin underneath!

This is usually used when the person knows it's ludicrous to say, but wants to share the story anyways, often to get it off their chest.