The red line indicates the trajectory as a result of understeer.
The understeer is a phenomenon that occurs during the driving of a vehicle that causes the actual rotation thereof to be less than that which theoretically should induce the position of the front wheels. In this way the front of the vehicle tends to come out of the curve. It occurs when the vehicle loses grip in the front end, which gives the feeling of "opening" in the corners. The opposite effect is the oversteer.
It is typical of front-wheel drive vehicles and is mainly determined by the weight distribution between the axles of the vehicle and by the inertia produced by mass transfers during driving. Also due to the fact that the steering wheels have to transmit lateral and longitudinal forces at the same time, since they are in charge of giving traction. Therefore, in front-wheel drive vehicles, understeer becomes more pronounced the more the accelerator is stepped on a curve. More information in English here: http://en.wikipedia.org/wiki/Circle_of_forces
In cars aimed at the general public, it tends to favor the understeering behavior of the vehicle in extreme situations, since it is easier to control by an inexperienced driver than the oversteer.
In understeer, usually caused by an abrupt turn, the front wheels begin to skid before the rear ones, so if the brake is slightly pressed, the transfer of weights from the rear axle to the front will help to increase the force of the rear axle. friction on this axis to improve the situation.
As an anecdote, the former rally champion and World Rally Champion Walter Röhrl once said: "Understeer is when you see the tree you are going to crash on ... Oversteering is when you only feel it."
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