Booming from above is the default, simply because you often get as close as possible, and you're out of the way of people and other obstacles. I know a few who prefer booming from below when possible as they like the sound it gives, and it can be very useful when actors are walking as you can point the null of the mic towards their feet, picking up less of it. Because of the tonal difference though between above and below, you should only use on or the other for a whole scene, or there may be difficulties matching the shots in post.
Maybe the mods could change the title back to what it should be? Would have avoided a lot of the comments. I must admit when I forst saw it, I assumed it was a foreign term I wasn't aware of. Never occurred to me it was a typo!
I recall a local sound guy telling me that booming from below puts the sounds of the body/clothing rustling etc., where applicable, out of proportion (i.e. louder) than the voice since the mic is now closer to the body than the face/mouth. Plus of course in an indoor environment you're pointing, possibly, right at the AC they won't let you turn off for the shoot.