CC adjustable BEC how does it work?

[no mods]

im considering using a lipo and CASTLE's adjustable bec on my 6 servo hanger 9 sundowner plane for RX power, with castles bad rap on components recently im kinda concerned and not sure if if it a a bad idea or not but my question is a little easier to answer. when using a say 3s lipo that naturally outputs 11.1 volts and the BEC is set to output 6 volts. where does the other 5 volts go...? heat? or is there something that im not understanding? this being said would i see a difference in say a 2s pack vs a 3s pack in terms of longevity if they were both the exact same MAH rating?

would a 1400 mah 3s lipo last longer than a 2s 1400 mah lipo.??

fill me in brothers...