Could daflu also mean to throw away (i.e. I'm going to toss the apple, it's rotten)?
Is "Daflu'r afal" a Welsh idiom meaning "To initiate something"?
No. You might as well throw a brick, a ball, a kipper or anything else that comes to hand!
That still leaves my to wonder what the phrase means.
It certainly can't be taken literally in any of the sentences that I've seen on DuoLingo.
No, it just means what it says, honest! Perhaps the problem is that the phrase given is not something that would ever occur on its own in the wild. In some sort of context you might hear:
Dw i'n mynd i redeg i ffwrdd cyn i chi daflu'r afal at yr heddwas 'na, Dewi Lingo - I'm going to run away before you throw the apple at that policeman, Dewi Lingo.
See the notes for this section.
The cyn i..., etc, patterns have no fixed tense - they depend on the context of the surrounding conversation to say whether they should be taken to mean 'before I threw', or 'before I throw, or ' before I had thrown', etc With this phrase on its own, any of those is accepted.
Why did Duolingo accept my "devour" for daflu earlier on in this exercise?