The act of sneaking up on an unexpecting sleeping person, of any sex, whose mouth is open whilst they sleep; and proceeding to slowly dangle ones penis into said sleeping persons mouth.
For a 'cranefly' operation to be successful the victim must not wake and the alton towers theme tune must be playing throughout.
Jazeem wanted some fun. He was listening to the Alton Towers theme tune so decided to go craneflying.
Tom was sleeping soundly when Jazeem snuck into his room on all fours, shuffled over Tom's sleeping form and lowered his erect jewels into Toms dribling open mouth. Jazeem enjoyed this for some time until sneaking back out into the night.