OK4Wire
Psycho is away at the moment I think, so perhaps I could give you the short answer til he gets back. Not a Tower person, so stand corrected if this is not the 100% version.
The stop bar lights have an automatic timer which returns them to the 'on' position a certain time after the runway controller switches them off, so that the default is 'on'.
I'm not sure of the time (45-60 secs) but basically if you are cleared to line up when you have reported ready still taxiing towards the holding point and the stop bar is selected off at the same time, there's the possibility that they may automatically come back on just as you are about to cross.
This has happened quite a bit and caused go-arounds when a preceding departure has baulked just as they cross the stop bar and now infringes the runway with no time to start moving again and roll in time.
To reduce the possibility of that happening, the controller may verbally clear you for takeoff/lineup then wait until you get closer to the hold line before turning off the stop bar.
There have been discussions about increasing the automatic timer on some of the bars but the advantages/disadvantages and safety risk need to be weighed up.
Hope that helps.