The spectator's freely chosen, signed, and notarized card is inserted half way into a shuffled, borrowed, government-inspected deck. The magician begins tapping rhythmically on the top of the deck and, just when spectators start to wonder if this is a magic show or the world's lamest drum solo, the deck sinks through the selected card, leaving it atop the deck.
There are a number of factors here that add up to one large factor, and that factor is the effect itself. The first factor is that the deck of cards is not really melting down through the selected card. In actuality (as the effect's creator Daniel "Macho Dan" Garcia likes to say), the selected card is rising up through the deck. The second factor is that it's not doing that, either -- at least, not entirely.
Here's how it works.
The magician allows a spectator to freely select any card from a freely select deck that was freely borrowed. (It is important that a borrowed deck is used, because the spectator will by writing on the selected card and it's always better to ruin someone else's deck than your own.) The spectator's name is written on the face of the card with a sharpie pen or, if convenient, wood burning set.
The magician now takes the card and apparently places it half way into the middle of the deck. But in reality, the magician tears the card in half across its middle (perhaps sneezing or coughing to cover the sound of paper tearing) and only places its upper half into the deck. The bulk of the deck hides the card's missing lower extremities, creating the illusion of a full card inserted into the deck. The card's remaining half is hidden somewhere convenient (such as in a sleeve or behind an unsuspecting young spectator's ear).
When the magician taps hard and firm on the top of the deck, the lightly held fragment of card will naturally jump from its resting place. By catching it a little higher in the deck and repeating the action many times, the illusion is created of the deck sinking down through the card.
Eventually the half of the selected card will arrive at the top of the deck. The magician traps it there with a thumb and, using the rest of the hand as cover, rejoins it with the all-but-forgotten hidden lower half using the standard torn-and-restored-card utility move (Barbell Course in Magic, volume XXXII, page 375, footnote 3, toward the end).
The card can then be freely examined by spectators while the magician subtly begs for tips.