Negative resistence never generates power. To create a negative resistence circuit, for example with a transistor, the power is supplied by power supply, not by the transistor. Transistor get power from there to create the negative resistance scenario.
To hill:
A negative resistence is the same as normal resistence, where ohm law applies right, but value of resistence is negative. Off course, is not a standard resistence, to create, you will need to use some special qualities of semiconductors, like triacs, or tunnel diodes, or gyractor circuits.
V=IxR where R is not a positive value.
It's mainly used in oscilator circuits, cause in a tank circuit with a negative resistence, it starts oscilating, cause it's unestable.