In any case, the elephant has a huge penis, and this is represented by a human males knee.
This can be taken as boyish behaviour act, as males will take their friends/mates by surprise in "elephant sexing" them.
Please see examples.
John, and Dave are walking along, when John then walks behind Dave and uses his knee in a thrusting fashion into Daves ass.
Thus creating the effect of having sexual intercourse with an elephant.
Person 1: "Ha ha I just elephant sexed you!"
Person 2: "Ouch that hurt what are you doing?"
Person 1: "You have just been shagged by an elephant!"