In a recent question on the Aviation Stack Exchange, one explained that would cause an autogyro's crash. Basically, it says that the one thing that would make the gyro crash is to unload the motor too much, as explained here:
[...] Unfortunately, as soon as the rotor stops spinning, the whole aircraft falls like a brick and the rotor may be impossible to restart in flight. This is a situation that should be avoided at all costs.
Normally it is not a problem since the weight of the aircraft keeps the rotor spinning. However, if the weight becomes too low or even negative, the angle of attack will become negative, and the rotor will slow down and eventually stop. It can happen when the pilot "pushes on the stick" and dives.
That's this bold text that caught my attention. I learned in high school that the weight of an object equals its mass in kg times the acceleration of gravity in N/kg. An object mass cannot be negative and AFAIK, neither does the acceleration of gravity. So how can it be possible that the weight is negative?