How to double undercut a deck of cards

Show the top card.

Cut a bit more than half of the deck off the top and swing it to the left using your index finger.

Clip the top packet under you left thumb and take out the bottom packet in your right hand.

Place the bottom packet on top and retain a pinky break.

Transfer the deck to your right hand retaining the break at the back with your thumb.

Cut about half away from the bottom packet and place on top, still retaining the break with your thumb.

Cut away the remainder of the bottom packet...

...and place on top. Square up the deck.

The King of Diamond is once again on top. :)