please help me analyze this sentence

[enydia]

Hi, Teachers.

I have some questions about the following sentence:

Anyone who has toiled through SAT will testify that test-taking skill also matters, whether it's knowing when to guess or what questions to skip.

I think the meaning of this sentence is clear, but is it grammatical?
What's the relationship between the two clauses? Why the writer used 'knowing' but not 'known'?

Thank you in advance.

Enydia *^_^*

[Tdol]

Knowing = having the knowledge

[enydia]

Knowing = having the knowledge

Thank you, Teacher Tdol.

But I still think the sentence is odd.

[Raymott]

Thank you, Teacher Tdol.

But I still think the sentence is odd.

No, it's not odd.
Anyone who has toiled through SAT will testify that test-taking skill also matters, whether it's knowing when to guess or what questions to skip.
Let's simplify it. I'll remove the first relative clause, and a few extraneous bits. (You'll tell me if I've removed anything interesting)
Anyone will testify that test-taking skill matters, whether it's knowing when to guess ...
Test-taking skill matters, whether it's knowing when to guess ...
"whether it's knowing" means "whether that skill is knowing when to guess ..." Does that make sense to you?

How about: "I hate farming, whether it's milking cows or harvesting corn."