Should your action points ever fall below 0, you will become fatigued. You will remain fatigued until your action points return to 0 or higher.
You may still perform techniques while fatigued, but they will be queued, and only come off the queue if none of your locations are busy. In other words, while fatigued you can no longer perform simultaneous attacks - you can only make one attack at a time. Furthermore, the delay of any technique performed while fatigued will be increased by 1.
You cannot enter a rage while fatigued. However should you already be raging when you become fatigued, you will not suffer any of the above penalties until the rage expires.
Your action points will continue to deplete if you attack while fatigued, but they will never drop below your inversed max actions (i.e., -2260).
See also: actions speed combat