Nope, none of you are right! Mise is kinda in the right direction though.
For the purposes of this question, it might be necessary to define "fraternisation".
Basically, whenever a Soviet heavy bomber went up on patrol, it would be almost as soon as it was in international airspace become escorted by NATO jet-fighters, usually American.
Since these flight were so long, the crews on either side would mess around with each other; using sign-language to communicate, taking photos, acrobatics, flying upside-down just above the cockpit of the Soviet plane, and showing them Playboy magazine.