Logic behind Financial roll-up from PS to PPM

Hi Experts,

In my task during play extremity activities erstwhile the strategy executes a Financial roll-up batch job, the roll-up programme reduced each the existing Forecast/Planned outgo values to 'ZERO (0)'.

Thereafter, the adjacent clip erstwhile strategy tally the aforesaid job; it brings/restore backmost from 'Zero (0)' to aged values (or aged values positive immoderate caller planned costs) from PS to PPM.

Irrespective of supra behaviour, I privation to cognize the modular behaviour of Financial roll-up program.

Does it overwrite the existing values straight OR marque existing values to 'ZERO' archetypal and past reconstruct backmost those aged values (including further values if any)?? 

Thanks successful advance.

Regards,