A ferrite works by going around a wire. The ferrite will then act as an impedence of a few ohms at the frequencies required to attenuate the signal - usually in the order of 100-500MHz or so. For greater affect the wire can be wrapped around the ferrite so it passes through multiple times. The effect of the ferrite is then increased by the number of times though the ferrite.
Ferrites like this are most often used to eliminate the noise of high frequency switching - not just the signal itself but the harmonics in the switch edge. This might not be from the PSU iself, it may be from whatever it was designed to be plugged into. I have had to use these on mains leads to suppress noise generated in the box under test. Ferrites are also used to prevent noise entering a unit via the cable.
These days any cable that doesn't come with RF chokes should be banned as unfit for purpose.
I must say I disagree - the design should be done well enough that noise is either not generated or the unit is immune to it. (Tell your boss that by making a minor mod to a PCB will mean you don't have to fit £2.50 worth of ferrite to a unit you make 3k a year and what his reaction.)
Yours
Neil