Sometimes in the past there has been an aircraft on finals! ATC ask you to expedite your departure as you enter the runway from the taxiway! Do you taxi out stop and line up on the numbers, hit the brakes and build to max power or start your takeoff slowly increasing power while in a turn to the centreline? to assist separation to the landing aircraft?
i have seen that done by more experienced pilots many times in large jet aircraft as well as small aircraft too! To be shunned on ? accelerating on the turn to the centreline? i.e. do you ever start your takeoff roll while entering the runway from the taxiway? Not advisable with max power so theoretically a reduced power takeoff
ATC would expect you to take off immediately without stopping on the runway. If you are going to do something else then you must tell them (which may waste more time) so better still decline the offer. All explained in CAP 413...you know... the RT Manual!