A geometric diagram that allows one to visualize the Poincaré formula for relativistic addition of velocities in one dimension is presented. An analogous diagram representing the angle sum formula for trigonometric tangent is given. ...He gives a very nice and simple geometric interpretation of the special relativity velocity-addition formula. I am surprised that no one discovered it before.
The paper also gives an accurate historical account. The velocity addition formula is maybe the most important 1905 Einstein discovery that was not already published by Lorentz. But Poincare had it earlier and Einstein did not have the geometric consequences.