paralell mono / whats the benifit

ive been lookin at several of my mcintosh amp manuals & they offer a paralell mono option but its not recomended.

whats the difference between paralell mono & straight mono & why does mac offer this option & then not recomend it.

is anybody running their amps in a paralell mono type configuration & what are the benifits & or drawbacks.

im confused by the whole paralell mono deal to begin with, i understand & use the mono feature on several of my mac amps but i sure could use an explaination of paralell mono.