Queen of Diamonds, and Four of Spades:
🃍 🂤

Your opponent has a hand with:

King of Diamonds, and Seven of Clubs:
🃎 🃗

You draw a card like this one:

Four of Hearts:
🂴

Now, look at this. You need a Seven to make a 21 in that hand of yours with a Queen and a Four. Your opponent has an incomplete hand containing the Seven you need, and you draw a Four.

Your action is to take the Seven of Clubs from your opponent and give that opponent the Four of Hearts you've just drawn.

The Four of Hearts will take the spot of the Seven of Clubs in your opponent's hand. You move the Seven of Clubs to your Queen and Four hand to make a 21 like this:

Queen of Diamonds, Four of Spades, and Seven of Clubs:
🃍 🂤 🃗

And your oppponent's hand would look like this:

King of Diamonds, and Four of Hearts:
🃎 🂴

King of Hearts, and Six of Spades:
🂾 🂦

And your opponent has a completed 21 with this combo:

Queen of Diamonds, Five of Hearts, and Six of Clubs:
🃍 🂵 🃖

You draw a card like this one:

Four of Diamonds:
🃄

Since your opponent has a 21 with a Five you need to complete your hand, you can't swap your Four with that Five since it's part of a completed 21 and cannot be touched. You would have to put the Four in another hand of yours.

Third, swapping a card involving Tens or Face Cards are a bit tricky.

Two of Spades, and Nine of Diamonds:
🂢 🃉
and you need a Ten or a Face Card to make a 21.

Your opponent has a hand that looks like this:

Ten of Clubs, and Three of Spades:
🃚 🂣

You then draw a card that looks like this:

Eight of Spades:
🂨

You can swap your Eight of Spades for your opponent's Ten of Clubs to give your hand a 21, and your opponent's hand would be the sum of 11, but remember, there's a good chance he would draw a Ten of a Face Card when it's your opponent's turn.

Your hand would then look like this:

Two of Spades, Nine of Diamonds, and Ten of Clubs:
🂢 🃉 🃚

And your opponent's hand would look like this:

Three of Spades, and Eight of Spades:
🂣 🂨

Two of Spades, and Nine of Diamonds:
🂢 🃉

And you need a Ten or a Face Card to make a 21.

Your opponent has a hand that looks like this:

Ten of Clubs, and Three of Spades:
🃚 🂣

You then draw a card that looks like this:

Nine of Clubs:
🃙

Even though you didn't draw a Ten or a Face Card, your opponent has an incomplete hand with a Ten card. You can still swap out the Nine for your opponent's Ten, but when you do, since the Nine cannot be paired with the Three, the Nine would have to be placed in another hand, starting with the hand that has an incomplete hand with one card of a hand that has a Two Card or a Ten or Face Card; but if not, an empty hand to begin a new hand. You need not place your swapped out card on one of your opponent's pallets with a hand that would have enough of a value to make it a 21.

You take the Ten of Clubs from your opponent and make a 21 with your hand with the Two of Spades and the Nine of Diamonds.

Your opponent's hand would be just the Three of Spades with the Nine of Clubs going to another hand of his.

Let's say you have a hand that looks like this:

King of Hearts, and Eight of Spades:
🂾 🂨

And you need a Three Card to make a 21.

Your opponent's hand looks like this:

Three of Spades, and Eight of Clubs:
🂣 🃘

And you draw this card:

Jack of Diamonds:
🃋

Your opponent has the Three you need and you have a Jack. You can elect to swap the Jack you have just drawn for your opponent's Three from an incomplete hand to make your hand a 21.

Your hand would then look like this:

King of Hearts, Eight of Spades, and Three of Spades:
🂾 🂨 🂣

And your opponent's hand would look like this:

Jack of Diamonds, and Eight of Clubs:
🃋 🃘

Queen of Clubs, and Three of Spades:
🃝 🂣

Your opponent's hand looks like this:

King of Spades, and Eight of Spades:
🂮 🂨

You then draw this card:

Jack of Diamonds:
🃋

What you could do is to swap out the Jack for his Eight in his imcomplete pile to make your pile a 21. Again, since his hand would just be a King and your opponent can't place the Jack with it, the Jack would have to go into another pile with one compatable card of the hand on the pallet. If your opponent otherwise has incomplete hands that have no Ten or Face Card, the Jack would go into one of them, but if not, a new hand would be created for the Jack to be placed on.