The way I have heard 'rolling' used, and the way I have sometimes used
it myself, is if given a clearance along the lines of "Cessna XYZ,
cleared for take-off, no delay" or "Cessna XYZ, cleared for take-off -
expedite, landing traffic".

In this situation, when you acknowledge the clearnace, you could say
"Cleared for take-off, Cessna XYZ". This does not confirm to the
controller that you heard the 'expedite', and so may cause the
controller to repeat the instruction. Or you can say "Cleared for take-
off, no delay, Cessna XYZ". Or "Cleared for take-off, rolling, Cessna
XYZ" or even "Rolling, Cessna XYZ".

As previously advised to in a thread a while back in this group, when
given a take-off clearance with an 'expedite' in it, I tend to glance
up the final approach path, then push the throttle in, then key the
mike for the acknowledgement. It's nice when controllers do things
efficiently, saving me 0.1 hour at rental rates, so I try to be as
responsive as possible in return.