This video https://www.youtube.com/watch?v=GhsZUZmJvaM https://www.youtube.com/watch?v=GhsZUZmJvaM explains it: they can change the attack angle of rotor blad

This video https://www.youtube.com/watch?v=GhsZUZmJvaM https://www.youtube.com/watch?v=GhsZUZmJvaM explains it: they can change the attack angle of rotor blades for different positions, which produces a torque, though you have to compensate for the gyroscopic effect, which lags behind. Humans cannot fly this, but computers can. The video also explains why the chose two contra-rotating rotors instead of a quad-copter design.