From a very reliable source:
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The Palms has taken steps to insure that "smart" players can no
longer hide wins by pulling their slot card when dealt a good
hand. The new system also won't let you pull your card after
downloading free play.
This is how it works. When your card is inserted and you put in
money or a ticket, a "carded session" begins. It continues until
the credits reach zero, or you cash out. If you remove your card
at any time in the middle of a hand, or with any credits
remaining, the card reader says, "Carded session in progress."
It is exactly the same as if your card were still in the card
reader. The "carded session" continues, even during/after a
jackpot which locks up the machine. The only way the session ends
is when the credits hit zero!
Putting in another card doesn't work, the machine won't accept
it. Says, "Card reader error." And even though you might leave in
this other card, play is still being credited to YOUR card.
Incorrectly inserting another card doesn't work either, same
message and result. I was also told that the card session won't
"time out" after a few minutes, as it used to.
Bad things: If you are used to deceiving the casino, this change
will not be welcome. If you are used to sharing a machine with a
husband/wife, girlfriend/boyfriend, or any other bankroll sharing
couple, you won't be able to switch cards without cashing out or
running out of credits first.
Good things: You can begin play with your card inserted, then put
it away. As long as the reader says, "Carded session in
progress" your card should be credited with your play. If you
have a large number of points, taking your card out will hide
this from passersby. On the downside, you might like seeing the
points actually being added as you play. You must remember that
if your credits hit zero with your card not in the machine, you
must reinsert your card when you put in more money, else
subsequent play won't be credited to you!
You can still have a single player's card inserted in two machines
and receive credit for your play on both machines.
Beware! Savvy con artist types might pick up on this. Say someone
puts a ticket in a dollar machine for $1.01, puts in his card,
plays one hand for a dollar, takes his card out and leaves. If
another player plays that machine WITHOUT cashing out the penny,
the carded session will continue for the PREVIOUS PLAYER! Any
subsequent card inserted will be rejected, so pay attention--
never play a machine without verifying that your card has been
accepted first!