You use part of the extract (I use half the 3lb bag of plain DME) to do the boil,hop additions,etc. Then,take it off the heat at flame out to do the late malt additions. It's already been through a boil,no need to do it again. A small amount is used for better hop utilization.
The remaining heat is enough to sanitize it wit the lid on while you sanitize the FV,etc. Then cool it down to pitch temp in an ice water bath.