IPC-7711/21 describes that underheating is especially necessary with thick multi-layer printed circuit boards that contain a lot of thermal mass (copper layers). You cannot solder on this PCBs without underheating. Of course, bottom heating is necessary for ceramic and metal core printed circuit board materials. A lot of heat will be dissipated in the copper there so it is impossible to solder without bottom heating (auxiliary heating). You can buy underheating (bottom heaters/hot plate) in different types: e.g. with infrared radiation (IR), or with hot air, or a combination of both (hybrid).