It's a single powerful projectile, that, when it explodes, triggers a fan of 40 reasonably-powerful hit-scans to originate from the player in the original orientation it was fired.
So, if you want as much of the shot to hit an individual as possible, you melee with it, being sure to keep pushing into whatever you hit after the big ball has exploded, because the hit-scans aren't quite instant (it's pretty likely to one-shot a Spider Mastermind. Particularly in ports like ZDoom that fix the blockmap).
To avoid wasting the hit-scans, you'll want to make sure that monsters are in the line of fire of them. As an example of how this works, if you fire the BFG at a Cyberdemon and then run around him (imagine he is the centre of a compass. You start at the South, facing in and then run around to the North as the BFG ball hits him) then all of the hit-scans will still fire off to the North (not quite reaching NorthWest or NorthEast, due to the arc), meaning the Cyberdemon doesn't get hit by anything other than the initial ball. If you were to start North, fire the ball off North and then run around to be South of the Cyberdemon he'll then get hit by some of the hit-scans, even though the ball was fired away from him (and even if you're looking South when the ball explodes).
Ok, thats really interesting and very well explained, thanking you very much :)
Obviously it is at it's most visually effective if you shoot it into a crowd of 41 Zombiemen that happen to conveniently be placed in an arc of roughly 80 degrees (I think) with one right in the centre to take the initial ball, but due to it's interesting behaviour, it is a very versatile weapon.
ummm, I think I may actually do that, be a great laugh. 40 hit scans seem like a lot, especially as you never usually see 40 enemies at once, but im guessing many of them will hit the same enemy for more damage if there is nothing else to hit. One projectile being powerful enough to kill a Zombie, hence as you say 41 zombie uses the weapon to best effect.