A power spell was a web cast or drawn to increase one's power with the gift. It also siphoned the power of other gifted, leaving them far from at their full strength.
The entire People's Palace was constructed in the shape of a power spell and worked to drain gifted, who were present at the Palace, of their power, while increasing the strength of any Rahl within the Palace. Gifted at the People's Palace would often find the loss of their powers disturbing, causing them to feel far more vulnerable than they usually would.