For everything except Doc's nair I think, moves are weaker the longer they are out. For example, Falco's nair hits the hardest on the first frame the hitbox is active and hits the weakest just before the hitbox disappears. This is also the case for moves with temporary hitboxes that disappear after only a few frames. An example of that is that the last slash of Link's upb is only strong enough to break a bomb if it hits during the first frame it is active even though it lasts only I think 4 frames.
There are also multiple hitboxes on most moves because each hitbox is a perfect circle. Sometimes, these hitboxes have very different properties. Half of Marth's dair will send you up and the other half will send you down depending on which hitbox circle intersects your opponent's hurtbox circles.
Some characters have decided sweetspots, where one hitbox does the same general thing as the sourspots, but does everything better, like marth's tipper fsmash or whatever.
Doc's nair increases in power the longer it's out, making it a very important exception to these rules.
Falco's fsmash has a weird hitbox inside Falco (not on his legs) that pops people more upward than the other hitboxes and can be used to extend combos.
Now for your original question, which I think was about how to do weak nairs and bairs. These moves do not have sweet and sour spots, so the only way to hit with a weaker version of them is to use the move and then hit with it during the later frames that the move is active.
I hope this was helpful and not too wordy.
