A force applied off a pivot does not just push — it TURNS. That turning effect is the moment (engineers say moment for structures, torque for shafts; the arithmetic is identical). $\tau = r F \sin\theta$, read aloud tau equals r F sine theta: $\tau$ is the moment in newton-metres, $r$ is the lever arm in metres — pivot to the point where the force acts — $F$ is the force in newtons, and $\theta$ is the angle between the arm and the force.

The sine is the whole subtlety. Pull square to the bar and $\theta = 90^\circ$, $\sin\theta = 1$, and every newton turns the bolt. Pull at an angle and only the perpendicular share does any turning; the rest runs along the bar and merely tries to pull it out of your hands. Pull straight along the bar, $\theta = 0$, and you get nothing at all — which is precisely why nobody tightens a bolt by pulling the wrench toward themselves.

Two habits worth building. First, the arm always beats the muscle: doubling $r$ doubles the moment for the same pull, and that is the entire argument for a longer breaker bar. Second, watch the arm's units — a bar quoted in centimetres dropped straight into the formula gives you a moment a hundred times too proud, and it looks perfectly reasonable on the page.