The air for pressurisation is normally supplied from the low pressure compressor side of the engines. At the start of descent the throttles are retarded so the engines are at idle. The pressure and indeed the volume of air supplied from the LP compressor would not be enough to maintain pressurisation with the engines at idle so there is an automatic valve changeover within the engine bleed system to supply the air from the high pressure compressor. It is not uncommon for a pressure "bump" to be felt during this change in bleed source.