You do it if something's rootbound (lots of roots encircling the ball) but don't want to use a larger pot, or to get rid of old dead-ish woody ones with few, if any new feeders on them (to fit into a new pot), or to get rid of slimy black rotted ones if you suspect a problem, or just as a matter of course when repotting to go into a smaller pot. You do it by about 1/3 all around like a shallow bowl, but that depends on the plant, and only if you're sure there are plenty of new feeder roots higher up near the trunk. For tropicals, timing isn't so important, though waiting til spring probably is a better idea in general. Ficus do better when very slightly rootbound (compared to other things), but if there is no real need at a given time, don't do it.

Thank you for the info! I will wait till spring.