helicopter
Explanation
The Joke
A man (shown in silhouette) is apparently performing the "helicopter dick" move -- swinging his penis in a circular motion -- presumably as a seductive or comedic gesture. A woman, looking unimpressed, responds with a technical critique: "I don't get it. Why isn't there a second, smaller penis to balance out the torque?"
The caption below reads: "Pro Tip: The 'helicopter dick' move doesn't work on engineers."
The Humor
The joke works by subverting the expected reaction to a crude sexual display. Instead of being impressed, amused, or disgusted, the woman reacts as an engineer would -- by analyzing the mechanical flaw in the design. In a real helicopter, a single main rotor spinning in one direction would cause the body of the helicopter to spin in the opposite direction due to torque (Newton's third law). This is why helicopters have a tail rotor: a smaller rotor that counteracts this torque and keeps the body stable. The woman is applying this exact engineering principle to the man's anatomy, wondering why there is no counter-rotating element to prevent his body from spinning. The humor lies in the contrast between the crudeness of the act and the clinical precision of the engineering critique, and the implication that engineers cannot turn off their analytical thinking even in intimate situations.
References
The comic references the torque reaction problem in helicopter design. When a helicopter's main rotor spins, it generates an equal and opposite torque on the fuselage (per Newton's third law of motion). The tail rotor (or anti-torque rotor) is a smaller rotor mounted at the tail that produces a lateral thrust to counteract this rotational force, preventing the helicopter body from spinning uncontrollably.