I believe he is referring to a probe from a thermometer, not the thermostat probe. The thermostat probe you can let hang in the middle of the incubator, or you can tape it to the Flexwatt. Then you can put a thermometer probe in the egg box. Some people do not put one in the egg box as it should be the same as the temp. inside the incubator provided you are using a good thermostat that maintains good temps. I prefer a thermometer probe in the egg box