Because you learnt the information/knowledge

But omniscient already know every thing

Exactly, that's exactly the same as you just known everything now, that's the whole point, it's like learning it but before the actual learning part, as if you learned it a lot earlier.

Because your mind is perfect and infinite . It's incomprehensible to put into words what it would be like .

So the omniscient knows how to use fictional abilities because he learned them?

Basically yeah, you can't know unless you learned it, it's like instantly learning as if you basically know already.