Forcing is a technique used for training with the One Power (however it can also happen by accident) to grow stronger in its use. It is considered dangerous because it requires drawing more of the One Power than one can knowingly handle. The benefit from this is that if the channeler is not destroyed or burned out by this process, they have quickly managed to increase their strength with the One Power.
It's mostly used by the Black Tower who think it the best way to get their army ready in lesser time. Those who die of this kind of training in the Black Tower are referred to as "training accidents." The method of forcing in the Black Tower is by making trainees do everything with the Power, including chores and cooking. The use of forcing is banned inside the White Tower. When caught as a damane by the Seanchan, Egwene al'Vere was forced. Forcing is less dangerous for women under guidance of a fully trained female channeler because it is possible for a trained women to know the absolute limit of another woman's strength in the one power.