The octave keys, or flick keys, are unique to the bassoon. There are octave keys on other instruments, of course, but they are part of the fingering, held down for the entire duration of the note. On the bassoon, the key is used only when initiating the note, to prevent a split attack, a brief multiphonic when tonguing, then released to avoid affecting the tone of the held note. They aren't necessary on most slurs, just tongued attacks. Also, there are three of them: one for A, one for Bb, B, and C, and one for D. Or that's the theory, anyway. Many students learn simplified fingerings which ignore these, and then must later learn, with great difficulty, to add them in order to advance. Some schools of playing ignore them entirely, accepting the split attack as part of the color of the sound. Then, there's how I first learned, when I was about 13. My teacher taught them as part of the fingering: that's how you play those notes, with those keys held down. His idea was that it would be easier later learn to release them after the attack, than learn to add them later. And its true, it's deeply ingrained for me, when I go to play one of those notes, my thumb goes to the key. My first bassoon didn't have a D key, so when I got one with a D, I had to learn to use it. It's still not as natural as the others, but I'm pretty good about getting it down.
Of course, now I have two problems. One is using them when it's not necessary. Slurred scales, I'm still going to those keys, even when there's no possibility of a split. This is a problem because it's wasted motion, and reduces fluency, as well as messing with the tone and intonation of the note. And getting off them quickly enough: to avoid the split, you just need to crack the hole open momentarily at the right moment, and then the rest of the note is unaffected. By default I'm jamming the key down, then start to think about releasing it. There's also risk: there are some nasty squeals hidden in a careless brush across the wrong key at the wrong moment. So I use those keys more often than absolutely required, and hold them down too long.
This comes up from the Mozart, the opening notes to the first trill section, which has a sixteenth-note D4 on the main first downbeat. I'm struggling with fingering this note anyway, since I've added the Eb vent to the fingering. But I'm pretty good these days at flicking it... good, right? No: M noted that at that speed, the D key is down for probably half the note, screwing up more than 50% of a rather important (but fast) note, and that D doesn't split anyway. I tried, and it's true: no multiphonic, even with no key. *sigh* So now I have to relearn again, learn to *not* flick my D's.