Learn English – What does the word “as-yet-unknown” mean

The phrase in the book is:

So they think you're dead, Andrews, and they have now played their last card. So we must hold ours back. You're going to remain dead—for a little while longer.

They're using an analogy to a card game such as poker. They have put their last card down, or made their intentions clear.

But they're going to keep their last card a little longer, and keep their secret from the others: that Andrews is not really dead.

'swish' is just onomatopoeia for the ball not touching the rim, only the net as it goes through, making a swishing sound. It's not a noun or verb. It's the word for the sound it makes.

The implication of the word is that Sheldon did something really well, something without any flaws at all. It is not a common thing to say, but is immediately understandable.

Another similar thing to say with the same meaning is descriptive "nothing but net" (much more common).