After the departing means exactly that.If the a/c ahead is not rolling it is not departing,You shouldn't go beyond the holding point until the a/c on the runway rolls.If the runway is long enough to have two a/c lined up then the controller would say line up rwy.... N0 2 to depart etc etc.Thats my understanding.I don't like to see a/c creeping on to the rwy whilst the other a/c has not yet rolled.if the first a/c for whatever reason takes too long and the controller decides to change his/her mind about being able to get the second a/c away ahead of an inbound he or she won't be able to cancel that conditional clearance,as it has already gone beyond the holding point.