Centrifugal compressors- blow off?

Hi, I would very much like to understand the operation of a particular centrifugal compressor. The compressors runs at full power usually, but for a day it throttles back to 70% power approximately. The compressor runs in tandem with a screw compressor(runs load/unload) for supply. I am thinking they may be blowing off the excess pressure in the centrifugal for a day? . is this an optimized operation. It would be of great help if I could get some info on this. Thank you.