A specific small amount of power is applied to the pairs. The pair that the return power is sensed on will determine which pairs will be used (no return or improper voltage/current sink and POE won't be turned up).
If power return is sensed, then another sensing phase begins. The client will return a certain percentage based on what it's power requirement is. The provider senses the percentage of returned power and adjusts to provide the proper power level (within the limitation of the device).
The last phase is providing the power at the requested level ... but the provider continues to sense the power drawn ... if the client attempts to pull too much, the power is shut down to prevent damage (to the provider, possibly to the client, and to prevent dangerous power levels and the possibility of fire from igniting an overheated cable).
This is a very basic explanation, but hopefully it give you the idea of what's happening.
Good Luck
Scott
