Forcing is a technique used for training with the One Power (however it can also happen by accident) to grow stronger in its use, as long as the channelers reach their full potential. It is considered dangerous because it requires drawing more of the One Power than one can knowingly and safely 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. Instead the use of forcing is banned inside the White Tower.
Forcing is less dangerous for women under guidance of a fully trained female channeler, because it is possible for a trained woman to know the absolute limit of another woman's strength in the One Power. When caught as a damane by the Seanchan, Egwene al'Vere was forced, but it seems that the use of the a'dam is a safer way to preserve the forced damane from damage.
This maybe is because the a'dam produces an involuntary circle between damane and sul'dam as was commented by Moridin. In fact it seems that the linking could be a safe way to protect a channeler who is forced. This is proved by recent developments inside the Black Tower, among the Dreadlords, where it is revealed that a man linked by a woman to another man who is teaching him, can learn weaves, improve skills and augment his strength in a faster a safer way than usual.