The main power drain of an electronic groupset is actually shifting, motors moving the derailleurs with enough force to shift the chain. When they’re not shifting and awake, the power draw is very low. They need to be awake to be able to receive any commands from the shifters. When they go to sleep, they shut off their receivers so as to minimize battery drain and need to awakened in order to shift. On SRAM, the slight motion on a trainer is enough to wake the deraileurs so that you can shift if needed. Of course in ERG mode, you won’t be shifting (much). I believe Di2 is similar.