Reachwash,
you seen the buffing machines in the shops polishing floors. Long handle, head on the floor about 20" where a polishing pad rotates.
same principle used for the rotary carpet cleaning but at a lower rpm, typically 150-230rpm max.
generally speaking, a pad goes on the bottom of machine that is then passed over the carpet, obviously water is in the process as is cleaning fluids.
the pad picks up the dirt from the carpet.
some refer to it as a "muck spreader" cause to some extent, whilst some soil in the carpet is lifted to the pad, some is transferred from one area of carpet to another.
sounds not good but the results can be very effective.