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This is not a technique. i think this is a requirement. In some advance communications like 802.11, because of the modulation type that create some peak to average ratio bigger than one. To make sure we can deliever the linear output power, we need back off to account for the possible peak happens during data transmission. For example, 802.11g, the typical backoff number is about 6-8dB. In this case, if you want to deliever 18dBm output power to the load, you have to actually design a PA as if it can support 26dBm output power, assuming 8dB backoff.
Power backoff is a method to get the required linearity for a PA. You basically over-design the PA's output power handling so that you can meet the linearity spec at a reduced power level.
Some PA's a programmable output power range. So there are more efficient for more backoff to there maximum output power. But these programming is stable over a full frame typical. So the distortion induced effects the backoff to the peak power capability is still valid.
Let's assume you meet your linearity spec at the peak power handling point of your PA (say, +40 dBm out). If your signal has a 6 dB PAR (peak-to-average ratio), you need to back off your operating point by 6 dB from the point. So, you need to operate your PA such that a CW (single tone) test signal would come out at (40 dBm - 6 dB) +34 dBm.
Your driving signal will have an average output power of +34 dBm, with peak excursions that make the PA put out +40 dBm for a brief instant of time. When the signal makes the PA hit that peak, you know that it will still operate in a linear fashion (low/no distortion).
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