In theory - PSR means that trains are dispatched on a schedule no different to passenger railroads. If there are 4 loads, you carry a train with 4 loads. Of there are 500, you carry 500. This is in contrast to how american RRs were operated on the "as needed" basis. It is not much different to how EU railroads are operated - as they need to share infrastructure with passenger trains, and thus need to reserve a slot over a given line.
PSR essentially means running a railroad like a clockwork, chasing as much of equipment usage as possible. There is a case to defend, that having locomotives idling without work because currently there is none, or the equipment pool does not balance itsself, causes bloat, thus cost. Likewise - I read somewhere that freight cars tend to see very little usage over their lifetime - mostly spending their time as mobile storage units.
In practice however, PSR degenerated to running as long of a train as slowly as possible either because of stupidity, equipment imbalances, general incompatibility of work culture with this approach, insufficient infrastructure or push of the management to save everything as much as possible.
PSR is trying to run an american freight railroad in a manner that japanese run their commuter trains. Japanese are running their schedules with 10 second ( or so ) precision because even a minute of delay causes a cavalcade of delays on the entire system. Try to run 10000t freights with such precision when management is breathing down your neck to save cost, infrastructure is crumbling, locomotives are barely operating and you are short on motive power anyways.