For FIM, you cut only about 80% of the sixth node off, leaving about 20% intact and on the plant. This makes the plant grow multiple tops, more than two. This is best when used in conjuntion with SOG, to keep the plants from getting too tall and insted making them bushier.
Strong Sativa strains respond well to topping. They want to grow tall and thin, but by topping them you force the bottom growth to fill in making a bushier plant. More Indica dominant strains on the other hand already want to form bushy undergrowth and a huge main bud, therefore topping is not as necessary. I know that Chronic is one strain that should not be topped in any way. Plants like these want to form a large main bud and topping ruins this plants production by stunting it's growth where the plant needs it most.