i'm not sure about recording in BIAB but in RB it's dead easy. you select 'punch in' for the area you want to rerecord, then wind back and then monitor the recording. when you get to the bit you want to rerecord, the mic becomes 'live' RB records over the top of the old recording and then reverts to simple playback after the record option has 'punched in'. no need for a count in as you hear the track and just sing (or play) when you get to the bit to replace.

i don't know if you can 'punch in' on BIAB